Is the "mold" soft and fuzzy or hard and crusty? If it's a soft, fuzzy growth, it is likely a true mold or fungus. It's a sign that the soil is remaining too wet. Try watering less frequently and improving air circulation near the plant. The mold may not directly attack the plant, but if the soil is wet enough to promote the growth of mold, it may also be wet enough to cause root rot to develop on your plant. If the "mold" is hard and crusty, it means you are watering too little at a time. The crusty material is likely a buildup of salts on the soil surface. Scrape off the crust, and change the way you water. Water only when the soil's surface is dry, and then water enough that it flows through the drainage holes in the pot. Dump out the excess water that collects in the saucer under the pot when the pot has finished draining. Avoid watering again until the soil feels dry. If the mineral content of your water is high, consider using collected rainwater or distilled water to help prevent mineral buildup.