Sorry but I find the advice/suggestions offered so far to be a little alarming and probably a bit quick without asking a couple of questions.
My first 2 are.......when did the animal last slough? And.......what substrate do you use?
Could just be a little damage from rubbing his nose.....could be ink/dye from substrate........could just be pigment change.........could be a bit of retained skin from the last slough.
Either way the pic's aren't that flash and while it could potentially be something more sinister lets try to examine the basics before we throw the OP into a frightened frenzy.