This is a Titan or Giant Triggerfish (balistoides viridescens). This is the triggerfishes bad boy. It can be more than 75 cm long. You'd better not play with it, because with its thick muscular body it can deliver a painful blow (when it rams you head-on), or it can bite you with its powerful mouth. And it does not need a provocation to act!
This beautiful and dangerous fish can be found from the Red Sea to South Africa, and in islands of the Central Pacific Ocean. It is one of the major predators of these coral reefs.