How often do you change the bedding? I'd suggest newspaper, but mold will grow on that, too, if it's not changed regularly. If you're changing it regularly...and still getting that much mold growing that fast...I'd be more concerned with finding the source of it. If there's no other mold issues in the house (especially near that tank) then maybe try bleaching the whole tank very thoroughly. Make sure it's completely rinsed and dried well before putting in fresh, clean substrate (newspaper really would be good to use).