That is odd. No spills or anything? Could you post up a pic?
I've safely cleaned the back of my white 5X with alcohol pads. You can get them at any drug store, they usually come in a box of 100 or so:
View attachment 208691
I use these to clean gunk off keyboards and other electronics and they work well. Maybe they might help here.
I'm using a clear case on my 5X now.