it depends on which butterflies.
the chevron and the melon butterflies will eat the polyps. but you can't see them. they have shovel like mouths capable of eating the polyps even when the SPS retract them. that's why they are so specialized in eating nothing but SPS. they are obligate feeders and have adapted ways in eating the polyps that other fishes cannot.
they don't eat debris and algae. they only eat SPS.
pakistan, millet seed and perhaps other species will eat algae and debris. but not the obligate corallivores.
yes they are always in pairs because these butterflies mate for life. they are monogamous pairs and will not split up unless one is captured or die, unlike more promiscuous species like raccoon, and other schooling species. another interesting thing to note is sometimes, the pair are of the same sex. but too laychey to explain why here.