In reading responses to the original question, I see a lot of misconceptions going on here. Mold generally needs three things to grow: food source, spores and moisture. The first two are everywhere because they are floating around in the air. Therefore, moisture is usually the issue that needs resolving. When we at Criterium-Cincinnati Engineers investigate mold growth issues, we are usually looking for the moisture source(s).
The moisture source could be due to many reasons, including some cited in the replies. But, the only way to really know the source is to have the issue investigated by a knowledgeable investigator. That being said, I will venture a couple of possibilities for the original mold growth. I doubt that the issue is a foundation leak if the location is above the foundation. Water usually travels downward--not up, unless it is wicking up in the materials, which usually takes a bit of water and the mold growth would be relatively heavy. If the found mold growth is on the lighter side, I am guessing that it is due to condensation because of its location behind the dresser. I have seen cases like this before. Mold likes dark, low-air movement areas in which to grow--just add water.
Mold can be cleaned up using bleach water or one of the cleaning products sold as being a mildewicide or fungicide (mildew IS mold IS microfungi, BTW). Be sure to follow the label directions. Also be aware, all "icides" need a certain amount of time in contact with the substance that is being killed to be effective, and that amount of time is usually around 10 minutes, assuming that it fully penetrates the stuff. To help penetration, you might want to add a few drops of a dishwashing liquid--my fave is Dawn (and not because P&G is located here). I also like Murphy's Oil Soap made up to be a little more concentrated than normal because it tends to coat the mold and keep it from getting airborne during cleaning.
However, if you do not eliminate the moisture source, the mold growth is likely to keep coming back. To prevent its regrowth, you might be able to relocate the dresser so the area is exposed to air movement to evaporate the moisture. Otherwise, you will need to take more aggressive measures to eliminate the moisture, such as better insulation.
However, you have to wonder, what is the source of the water in the home that is condensing in the area. A lot of water is generated in the home by cooking, bathing, people, etc. Many homes have these sources without condensation; so why is condensation occurring here? Sometimes, the issue can be more insidious, such as water pooling up the home. That is where the pros come in.
Note that I have focused on condensation as the moisture source causing the growth. Other moisture issues can definitely be at fault, such as water not weeping properly from brick veneer, leaks around windows or even from roofs, etc. I once investigated a leak that occurred on the second story but only showed up in the basement. Again, that is where the pros come in.
Hope this info helps.