360 Xbox 360 Controller Storage

[xhunter]

So, I have a rather large collection of Xbox 360 controllers. I was cleaning/arranging everything last night and noticed that some (especially the older ones) of the controller thumbsticks had a white/gray coating of something on them. It wiped off easily enough, but it seemed like it might've been the rubber degrading. Is this a common issue (that can or can't be avoided)? Could it be due to me having cleaned them with a little bit of alcohol months ago and that caused the top layer of rubber to degrade?