Okay thank you for that input thats what I was afraid of is ruining it by cleaning it incorrectly. I'm also afraid though of it getting more ruined as mold and mildew spread. It was the hospital that did it, their child life specialists and honestly they didn't seem that confident in their knowledge of the product. The heart shaped one that is forming the mildew was from another kit like just one of those kids ones you can buy to make a hand print with just a clear plastic mold but they tried it and the mixture didn't work so they got the one they used to make the cast were just like "hopefully this works" and used it in the heart shaped mold. could there have been a reaction from some cross contamination? The cast turned out fine (so far) and it was the same material. I have emailed them to see if they can provide any info.