How healthy do your tomato plants look?
Mold thrives in very moist conditions. Is it possible you are over-watering your plants? Tomato plants should be watered when the first inch or so of soil is dry. Scrape off the mold, discard the mold and let the soil dry out a little.
Have you ever experienced blight on your tomatoes before and if so, were those tomatoes in these same pots? The only other thing I could think of is "southern blight" - that would look like a white mold on soil. However, I do not think it is likely in this case.