Could have injured itself squeezing into a rock to sleep or bumping into a rock...etc. I have this happen with my tang's and rabbitfish every now and then. I let them heal naturally and just feed them well.
It looks like an injury to me, or perhaps a bacterial infection. Whichever it is the redness definitely indicates infection. @melypr1985 what do you think about just trying some kanaplex-infused food?