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You can hand feed them. Put some frozen food on the end of a stick. You'll be amazed at the size of the food pieces they can eat! Their head kind of splits down the middle and expands 3x to fit over the food.
I've learned a bit about these guys of late. Certain of them can be damaging to corals. You'll catch them in the act because they have to attach themselves to the coral for quite a period in order to "eat" it. (it's more like sucking it to death).
The harmless ones tend to be longer, thinner and stay in one place. The harmful ones tend to look larger, thicker, have bushier white tufts and freely roam all over the place.