I'm wondering if your soil is too heavy and retaining too much moisture? It sounds like it if it is actually mold on the soil. Is it kind of fuzzy looking? If not, it may be accumulated salt crystals. I copied the following from the eHow website:
How to Remove Mold From Dirt Mold Prevention for Indoor Plants
Things You'll Need
Clean, empty spray bottle
Distilled white vinegar
Water
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Instructions
1
Make sure the mold really is mold. Sometimes salts and other dissolved minerals can leave a white residue on the top of your soil which looks like mold. From a distance, the two look similar, but upon closer inspection, you can see that the mold appears fuzzy, while the residue looks more crystallized.
2
Identify the cause of your mold. Most cases of soil mold are due to overwatering. Mold requires a constantly moist environment to grow and thrive. A good rule of thumb for houseplants is to water when the top inch of soil has dried out. This means that the soil below this one inch is still moist, but has begun to dry out. This watering schedule will keep mold from having a consistently moist home. Another cause of mold is high humidity. If your home has excessively high humidity, try a dehumidifier, or add a fan to the area to circulate more air.
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3
Use vinegar to kill the mold. Mix your spray bottle with equal parts water and distilled white vinegar. Spritz this solution on the soil of your houseplant, covering the entire soil surface. Do not drench the area with the vinegar spray. You want to lightly mist the soil in about one to three passes, depending on the size of the pot. The vinegar changes the pH of the soil and kills the mold very quickly.
4
Retreat the soil. If after three days, your mold isn't completely gone, mist the surface of the soil again. Soil mold is often killed with one treatment, but persistent soil mold may require two treatments. Also, retreat any time you see more mold reappearing.
5
If after several treatments, your mold is not going away, consider using a spoon or small shovel to remove the top layer of soil which contains the mold. You can just scrape the soil off that has molded, or remove the top half inch and replace it with fresh mix. It is important to note that mold will continue to grow until the
