Your case, could be mold, and it's not about the temperature but more to humidity. Yeah, partly due to our weather but it's not just about temperature. If your figures are in a case or detolf, just add the dehydrating packets (the one u see in bottles of medicines or vitamin) inside your detolf shelves or in figure box, if you keep figures in box.

The issue i have is greyish lines or dots which looked like dust fixation or paint scratch, mainly on my older figures. My new ones no longer have them, mainly because i keep my stuff in box more, i only take them out once in a while.