If you are talking about the standard longnose butterfly that is yellow on the back end and white on the face, they are designed to eat critters from between stoney corals. In captivity they will eat pretty much anything once acclimated to the tank. You want to be sure to supply them with meaty foods though. JMO.
Mysis is fine. Variety is best so offer some finely minced clam and mussel from time to time. They have specialized feeding requirements due to the extremely small mouth so anything you give should be a fine consistency.