Yes, you hit it on the head. The long snout is for eating coral polyps, Aiptasia Anemones and other polyps extending out from holes in the rock. All kinds of worms are also on the Butterflyfishes menu. Thanks for asking the question and for being a responsible hobbyist.
There are many Butterflyfishes in Hawaii, but I found this tide pool which rarely saw Butterflyfishes(or Tangs). Note the Aiptasia population.