Pale anterior part, elongated and oval body, whitish tail with dark blotch, prominent horn, small protrusible mouth. This is definitely Naso brevirostris, The short-nosed unicornfish.
It's a unicorn fish, also spelled unicornfish, any of certain exclusively marine fishes belonging to the genus Naso, in the family Acanthuridae (order Perciformes), occurring in the tropical Indo-Pacific region. The 17 species are herbivorous algae eaters. Unicorn fishes have a pair of sharp forward-pointing spines that protrude from the side of the tail shaft, and the fishes also have a long spike, a frontal horn, protruding from the forehead.
The bluespine unicornfish or short-nose unicornfish (Naso unicornis) is a tang from the Indo-Pacific. It occasionally makes its way into the aquarium trade. It grows to a size of 70 cm in length. It is called kala in Hawaiian, and dawa in New Caledonia.