Thats one ugly fish. Did it hale from HI? If so it could be an achilles/yellow hybrid which is common although the face doesnt really fit that scenario.
Meh, it's not a hybrid. Ctenochaetus and Zebrasoma tangs are known to develop aberrant color patterns as a result of experiencing skin fungal infections during the early part of their life. When the area of the skin infected by the fungus heals, the pigment cells are permanently damaged and the skin 'scars' a different color.
yellow tang