We have a 12'x20'x48" quick set pool.This is the 3rd summer we've set up the pool and all of a sudden there's mold/mildew type growth on the top of the inflatable portion of the pool. We've tried multiple cleaners with no results. The inside of the pool is beautiful. The problem only exists on the top; nowhere else. Any suggestions? There hasn't been a cover on it to trap moisture so I was thinking it must be something other than mold/mildew. Has anyone else had this problem?