I know exactly what you're talking about! I'm 
pretty sure I've seen this described as "paint 
lifting" or even just "lifting".

I have a few bits of it here and there in my 
collection, but I've never tried to do anything 
about it. Mostly because I'm also unsure exactly 
what can be done about it. . . plus it isn't very 
noticable and generally doesn't get worse. Most 
of the time, I think it's caused by a cel having 
been separated from something to which it was 
stuck.

Many Sharp Smiles,
--Drac