I had the same prob last week. I'm no expert or anything, so this is what I did, try at your own risk:wtf:

When temps are cool, encourages mold to grow. I dropped my lights, quit watering, got temps up to 78, humidity 30-35% range.

With the up temps, I turned off fans next day and removed the dried out layer of topsoil from around plants and threw away.

I have kept the temps up a little higher than I usually do since then. After transplanting I have not seen any return of mold.

I am using 24-0 right now so I used dropping light to raise temps.

Hope this helps. I hate mold.