Its a juvenile Sailfin tang. Quite hardy n easy to keep, once it starts eating, n it usually will eat anything n everything. Try out all the diff types of marine pellets, frozen shrimp, frozen brine-shrimp, frozen bloodworm, nori. Soak in Entice, or garlic extract.

In future id suggest u research/id ur fish b4 buying, n ensure its eating. I always ask the LFS to feed pellets or frozen food to show me its eating healthily.

Happy feeding