feed the butterfly anything that it wants to eat. they are very difficult to get to eat prepared foods. they will however clean your tank out of the small
feather dusters though.
mysis shrimp seems to be the most common prepared food that they will eat though, so that is a good starting point.
it will not compete for food with the mandarin. the copperband tends to eat polyps, like aiptasia (if you are lucky). they also like small worms, hence the reason they will clean a tank out of feather dusters.
G~