Looks like "bag rub", which generally happens during shipping, however, if the fish isn't new, your fish is either rubbing its mouth against the tank wall (begging maybe) or may have smacked it on the rockwork, but it looks more like "rub" to me.
This generally heals up all on its own, but keep an eye out for signs of secondary infection (redness, swelling, etc).
Thanks dude, I have had him for over a year. Interesting enough I noticed the same thing on golden trevally. I will keep an eye to see if it gets worse. Just in case, if it does start to get red, what should I use to treat?
If it gets red, swollen, or the adjacent areas become streaked with red, it generally means a bacterial infection has set in. You'd want to use a good, broad spectrum antibiotic such as Furan 2 in a hospital tank.