it sounds like rather than mold, you have a case 
of trapped condensation/grease that went bad. you 
can try to clean it if you can slide thin strips 
of paper in between the layers to pick up the 
moisture. 

I think cels do occasionally get a sort of mold, 
because I have a cel with teal spots between the 
cel paint and the acetate surface. if it's mold, 
the spores must've gotten in there as the artist 
was painting, because it's airtight now and I 
can't justify it otherwise. blech, gross.

/mina