This is an official Hori TPU case that for some reason seems to accumulate a weird residue on it after about a day or two sitting in its case. It's kept in a cool, dark location which isn't damp or anything, and neither the screen nor any of my other devices show any kind of build-up.

It's not moist and requires a decent amount of pressure to rub off -- I've taken to washing it with warm soap water which works, but it only comes right back after a couple of days. Does anyone know what this is? For the life of me I can't find any information on it, and it only appears on the outside of my 3DS case (the actual system has nothing on it).

Any thoughts? Thanks in advance.