Some urchin will eat corals. However, your urchin being on top of a coral doesn't mean it was eating that coral. Urchins often end up on top of corals for the same reason that they often knock off rocks--they couldn't care less where they travel. They "walk" over everything, sometimes knocking over a rock, sometimes stepping on a coral. Now, if you can find evidence of the coral being eaten, then move that bugger to the sump, otherwise I wouldn't worry about it. I have one (a pincushion) that has been a model citizen, except I THINK he took a taste out of my cabbage leather once, but that is just a hunch, cause I saw him nearby when I noticed the damage. And, yes, urchin will also eat coralline and detrius, and some can be spot fed with frozen food and such.
