There are two kinds of longnose butterflyfish in Hawaii, the common longnose and the big longnose. Neither name is especially flattering, but I think this is a big longnose butterflyfish. It feeds mostly on small shrimp, which it catches by thrusting its long nose into small crevices and swallowing its prey whole.
The little bright-eye damselfish on the left might have been startled by the butterflyfish, but not threatened. They can be quite aggressive in defending their small territories.