These fish are notorious for being very finicky eaters. They are considered to be one of the reef safe butterflies but it's not guaranteed.
I have one in quarantine right now (second day in there) and it hasn't shown any interest in eating.
I'm tring a variety of foods such as PE mysis, frozen brine, cyclopeeze. If he doesn't eat today, I will have to try different foods starting with live brine.
Some has stated that they have had luck feeding fresh mussels, clams, and live black worms.
As I stated previously, these fish are 50/50. Do a search through the reef fishes forum and you will find hundreds of threads anwering this same question.