Powdery white?  You know, I thought that my Night Below was just gathering tons of dust.  Interesting - can we get word on waht is causing this?

Also, I found that the worst-quality plastic was in Blood War.  Stuff from that set breaks if you look at it funny.

I've noticed this for the first time several months ago, and I was reminded of it when I got a night Below mini that had this while it was still in it's bag. Most minis in Night Below have this problem, although the black dragon has it as well. I don't think it has a lot to do with storage, since there are several sets in one cupboard and almost all Night Below minis have it but almost none of other sets. One thing that might have an effect is ventilation (or lack of).

One thought that just occurred to me is that maybe it isn't related directly to the plastic either, since it als accumulates on the paint albeit to a lesser  extent. So what is it?

creatures in bags have it so it must be production process related. It gets worse over time so that suggests some sort of (chemical?)  process is going on.

By the way, it DOES rub off. Not entirely, the plastic stays a bit greyish.

My best guess is that it is some kind of Primer related dust that in a closed environment settles on the minis, where due to color it is most obvious on the black bases. Note that this wouldn't be a very good thing...
Second best guess: substance used to clean the bases goes white after some time.

Completely wild guess related to the second guess: the kind of cleaner used is biodegradable and that is exactly what is happening, it's degrading.

edit: the reason I suspect the cleaner is that on some minis it seems to form patterns that resemble wipemarks