Welcome Mr. Jones:
The water line comes in from underground into the warm, possibly damp air of the basement which will cause condensation. Before I would call anyone, I would clean the pipe up, dry it off and watch for a leak. If you check it in the morning when everyone is getting ready for work you will probably find the whole pipe wet and running down, giving the appearance of a leak.
The little bit of corrosion on the union could be caused by the plumber's flux when he made the solder connections. If it is a leak, it will spray or, at least, dribble fast from one location.