Cellulose is plant fiber. :o) Celluloid is what's 
used in animation. Although there *is* such a 
thing as cellulose acetate, which is used in the 
manufacture of yarn, some photographic films, and 
as a celluloid substitute... hrm. But, yeah, when 
museum people talk about cellulose, we're usually 
thinking of plant-based organic materials, like 
fabric and the like.

Anyhow, the best way to keep it from spreading is 
to keep it in as dry a place as possible. 
Ordinarily, you'd also want to keep it in a hot, 
dry place for optimum benefit--- but the heat 
will age the acetate. That's the problem when 
dealing with items that are made of multiple 
materials...

There's no way to remove the mildew without 
harming the acetate. Mildew is a very tenacious 
little thing, and once it's on, it's difficult to 
get off, especially without chemicals. And while 
you may be willing to kill the mildew, you 
probably don't want to destroy the cel in the 
process. ;o)

Your best chance is to just keep it from 
spreading by depriving the mildew of the 
conditions it likes.

Good luck,
-Cres