I have a portable pool. It was up for a year+plus then we moved, so we took the pool and plopped it in the new yard. Under the pool I have a layer of Happy bottom and some other foam too. when i rolled it up to move it smelled awful.. It was just the rotting grass smell. I put it back down at the new house. I can't remember if i washed it off or not. Maybe. it was a natural decomposition thing. The point is I cannot smell it now. it's under 80,000 lbs of water. nothing is getting out from under the pool now. So, If mold were growing were growing there would you care?, Most of the mold problems in home are the molds that grow on wet gypsum board that feed on the paper. I'm no expert but i have been around above ground pools all my life, never heard of a mold problem.