Mold is a fungus and there are many typse of fungus that can develop in soil. However, all of them require fungus spores before they can become established. If you use soil as opposed to potting mix, then you are more likely to have spore contamination.
Scrape off the mold and the loose soil surrounding it. Do this repeatedly as new mold appears and you should gradually see a decline in the mold growth. Try to keep the soil as dry as possible. Warm temps and good air circulation will also deter mold growth.
Good luck!
