Ouch! White Cheek Tang has an accident!

[GreenSpottedPuffer]

Jeez thats deep. Poor guy. Hopefully he can get some food to help him heal. The only thing that comes to mind for white tufts growing on the fish is Lymphocystis but it doesn't really look like that from that picture. "Cauliflower disease" does not have a cure but usually clears up on its own with good water conditions. I know your tank has great parameters too, so i have my doubts that is what it is but you may want to do a quick search on google and try to rule it out. I had a puffer who got it a few times even in a healthy tank and it went away each time. Never really figured out why he would get it though.

Good luck!