Potting soil can grow mold if it receives too much water. Bagged potting soil with mold growth on it is due to the bag being unsealed and the soil exposed to moisture. Moldy potting soil in a pot results from over watering or high humidity.
While mold in potting soil is often harmless, it can be unsightly and may indicate there are problems with the soil. White or yellow mold on the surface of potting soil is saprophytic soil fungus, which feeds on decaying matter. This mold can easily be removed in potted plants by scraping it off with a spoon. Be sure that all of the moldy soil is scooped up and not mixed with any of the remaining good soil. Misting the now mold-free area with a mixture of equal parts water and vinegar helps clean the area. Avoid saturating the soil with this mix as vinegar can hurt plant roots.
If there is concern about excessive mold, repot the plant in new soil. It is also a good idea to clean the pot that holds the plant to avoid any cross contamination. This can be done by washing the pot in a solution of one part bleach and 10 parts water. Rinse the pot extremely well and dry before adding fresh soil.