What sort of plastic are cels? I've noticed that 
non-archival cel bags -- even some "propylene" 
bags -- will have the similar "dusty" look that 
I've seen of scans of old cels. Cels weren't made 
to last forever, so I doubt that the plastic's 
archival. It's possible that as with a lot of non-
archival plastic, the white stain might be caused 
by breakdown of the plastic and the release of 
subsequent acids on the material. Not too sure 
about this. 

I doubt the dust is caused by water stains -- 
unless the water was hard and with a lot of 
minerals. When the water evaporated, minerals 
would be left to collect along the side of the 
cel. Alcohol, unlike water, will not pick up all 
of these minerals, which may explain why the cels 
are still dusty, even after washing them. In this 
case, have you tried distilled water? (WARNING --
I don't have any dusty cels, and I'm not sure 
what the effect of water will be on cel paint.)