This spectacular fish actually starts life as a drab female and as they mature some of them change to these remarkably patterned males.  Like all Parrotfish, they have a parrot-like beak that grinds the surface of corals - sometimes scraping off live coral but often shaving the fine layer of algae that covers most non-living surfaces on the reef.  In this way they act as key "lawn mowers" keeping corals from becoming overgrown with algae.  As they eat they take in bits of hard coral which is ground up in their gut to be excreted as the white coral sand we love to swim above.