What's going on with this oto?

[Caol Ila]

This chap has had this white mark on it for a little while now, well over a week. I think it's an injury but I'm not sure. It's behaving normally and no other fish have anything similar.

[plankton]

How old is it?
They do go almost white as they age, I had one that was completely white after 4.5 years. He started changing at around 3.

I can't see any damage on yours.