Wish I could see a picture of the actual mold. Are these plants grown outdoors? How is the soil moisture? If you say that your plants aren't overwatered, and they live indoors, then chances are you don't have a mold. If they live outside, and it may have been raining frequently, then you might have a mold. Aslo, you may want to check for bugs. If you notice aphids and/or ants, you may have a sooty mold. Do you notice any mold on the leaves? Need more info and pictures would help greatly.