I'm raising this topic, since just couple days ago I noticed that my Green Cobra Ninja had this "powder" like stuff all over his extra head that I have kept in the box with the other extra stuff. Though it is not really like chalky powder, but more like a grey dust that seems to have kind of oily feel when wiped. It could very well be the same issue, and I don't see any reason to start a new topic conserning the same kind of a problem.
I have noticed something a bit similar (yes, some of that is just a dust) with those Joes that are in my detolf, but this one that was kept in the box had most of this stuff on it thus far. Now I remember that we had similar topic some years ago, about the head of the Stalker-figure. Now this all would indicate that there is somekind of a deterioration going on. A cheap PVC-plastic can start showing symptoms like this. No one really knows what could directly cause it, but a substandard pvc seems to be a really vulnerable.
I found this bit from the bowels of the net:
The semi-flexible PVC that Toy Biz used for the majority of their LOTRs toys was pretty much standard stuff in the toy industry. Most of my figures have held up quite well, but I am pretty careful about storing them in a climate controlled area. Some of the stuff I have bought over the last few years, though has actually been degrading faster than older items. Most of the JLU figures I have bought as well as the DC Super Friends (both made by Mattel) have had severe chalking issues. This is a issue you run into with really cheap Polyvinyl Chloride (PVC). Chalking is the deterioration of the vinyl surface, thereby exposing the TiO2 particles to the surface. The white powder chalking that can be wiped from the surface of most vinyl is TiO2 particles mixed with deteriorating vinyl (PVC). TiO2 is titanium dioxide – a base pigment used in the manufacture of most PVC.
The question is; Will those parts last? I haven't noticed that those parts would have become more brittle or anything. Though some of those part are indeed painted, and it shouldn't happen to painted PVC parts.
Room temperature (in my house) should be normal and same goes the humidity. Besides it is namely heat, cold and UV light that can cause those issues above.
Quite annoying that toys this expensive can have this kind of an issues. Not even my old MOTU, Lego, TF and M.A.S.K toys have anything like this going on.
Edit: I can't say that it is 100% sure, that this problem is namely the PVC deterioration mentioned above. Though it most certainly isn't just dust. Vitrines are not airtight, but in this case there shouldn't be any dust like that inside that box and in my case this "plagues" only the Sideshow Joes.