I'm not sure this applies, but I was getting a bit of white mold on top of my soil, on my indoor plants and I looked it up and it said on the net that it was pretty much normal, just means that the soil is moist, like you said. As long as it doesn't start attacking the plants I think you should be okay. Perhaps you are watering a bit too often? I'm letting the soil dry out until the top crust is visibly dry and that seems to work fine. Just my 2 cents.