The most obvious cause of a whitish discolouration on any kind of grouted ceramic wall or floor tiles would be a grout haze that wasn't removed after grouting. That discolouration wouldn't disappear completely when the tile is wet, but would be much more apparant when the tile is dry.
Now, it IS possible that you simply have a residual grout haze on the tiling, but I would expect that the tile setters that did that floor would have known how to remove that grout haze and would have done that as part of the job. I'd try scrubbing the floor hard with a very mild abrasive like a white nylon Scotchbrite pad (or even a worn out nylon stocking) in an inconspicuous spot to see if it does any good at all. If it does, then disregard the rest of this post, and talk to some ceramic tiling contractors on how to remove grout haze that has cured on the surface of the kind of tile you have. On glazed wall tiles, grout haze is harder to remove once it's fully dried, but it can still be done with a relatively soft abrasive, like the green 3M Scotchbrite pads sold in grocery stores for scouring pots and pans.
I'd presume the problem is grout haze and try removing it with a mild abrasive (like the white Scotchbrite pad) and even using a mild acid (like CLR cleaner) in an inconspicuous spot.
However, if you find that nothing you do seems to remove that "grout haze", there is something else that would give you a white discolouration when the tile is dry, but that would disappear when the tile is wet, and that is if the surface of the tiles were roughened somehow with very fine pits or scratches.
The whitish discolouration you're seeing could be due to a rough surface on the floor tiles, and that could be for any number of reasons. (It could be that someone used a sanded grout on the soft floor tiles, and the sand grains left the surface rough with scratches, it could be that someone tried using an abrasive like steel wool or a fine steel bristle brush on the floor trying to remove grout haze, or it could be that someone decided to "clean" that floor with a sand blaster, or for 100 other crazy reasons that would end up with the floor having gazillions of tiny pits and scratches on it.)
But, if there was something white on the tile that needed to be removed, getting the tile wet would not cause that white "stain" to disappear. It will, however, dramatically change the behaviour of light that hits the tile, and the way that light reflects off the tile. And, of course, changing the way light behaves affects what you see, and could easily result in a white discolouration disappearing.
Lemme first explain why a roughened surface could cause a white discolouration:
The reason why surface roughness will show up as a white discolouration is because each scratch in the floor is rough, and those scratches both reflect and refract incident light. The result is that incident light is scattered in all directions by the scratches (or other surface roughness on the tiles), and your eye sees that scattered light as the colour "white". Thus, your floor has a whitish appearance for exactly the same reason that clouds, waterfalls, snowbanks and the head on a beer are all white in colour, even though nothing inside any of these things is actually white in colour. What you're seeing is the scattering of light, and your eye sees scattered light as "white" light. So, your floor has a whitish appearance because of it's rough surface, not because of any substance that's on that surface that needs to be removed.
Now, why a thin film of water on the tile would cause that white discolouration to disappear:
The reason why that scattering effect disappears when the surface is wet is because of something called "Fresnel's Law", which says that the fraction of light reflected from an interface is determined by the DIFFERENCE in the refractive indices across that interface. The "refractive index" is a measure of how much light bends when it passes from one media into another.
Fresnel's equations predict that if R1 and R2 are the two refractive indices on each side of an interface, the amount of light reflected from that interface would be proportional to:
(R1-R2)^2/(R1+R2)^2 or ((R1-R2)/(R1+R2))^2
Or the difference in the refractive index divided by the sum of the refractive indices, all squared.
In the first case, where the tile is dry, then the refractive index between air and a solid like ceramic tile is going to be large, so the difference between R1 and R2 will be relatively large, and much of the incident light will be reflected.
In the second case, the difference in refractive indices is going to be greater at the air/water interface than it will be at the water/tile interface, (because the density of water is closer to that of a solid than it is to that of a gas like air) and so MORE of the light is going to reflect off the air/water interface, and much less of it will reflect off the water/tile interface. The result is that the amount of SCATTERED light that you see coming from the water/tile interface will be much less when the tile is wet, and so you will see much less white light when the tile is wet. Also, since most of the reflected light you see will be from the air/water interface, the surface of the tiles will look SMOOTH, the way the tiles were probably meant to be.
Here's more info on the general form of Fresnel's equations:
The above simplified equation only applies when the incident light ray is perpencidular to the plane of the interface, so all the angles are zero degrees. However, the simplified form makes it easier to understand why the amount of light reflected from the surface of the tile is very much less when that tile is wet as opposed to when it is dry. And that, in turn, explains why the whitish discolouration would seem to vanish when the tile is wet.)
Your best bet would be to treat the problem as grout haze until your efforts prove that it's not grout haze. Grout haze is caused by gillions of tiny particles of grout cement that has dried on the surface of the tile. Grout haze can typically be removed by scrubbing or with a mild acid, and experienced tiling contractors would be your best source of info on how to remove it from the kind of tile you have.
If it turns out that the white discolouration is due to surface roughness, then about the only option available to you may be to either apply an acrylic grout or masonary sealer to the tile to give the tiling a permanent "wet look", or take the matter up with the builder and ask him to fix it or adjust the price you're paying. Explain to him that since the discolouration is only there when the surface is wet, that proves that there isn't anything white actually on the tile, but that the white colour is being created by the scattering of light, and that is due to millions of scratches (or other surface roughness) on the surface of the tile which shouldn't be there.
It's possibly because the workmen who were installing the tiling may have had tile dust from cutting the tiles imbedded in the soles of their shoes, and the result was that the dirty shoes scratched up the tiling. Or, perhaps the floor was dirty with ceramic tile cutting dust during the time the tiling was done, and the hard sharp dust particles were ground into the floor tiles underfoot, thereby causing the surface roughness.
If you seal the tile with an acrylic "film forming" sealer (instead of a penetrating sealer), you will give the tiling a permanent wet look to remove the whitish discolouration, but you'd also have a floor that would be slippery when wet. You might consider sealing the tile, but using a bathmat on the floor in wettest areas to keep it dry in those areas and safer to walk on.