Algae (I hear they may eat coralline, though I've never seen him eat it). It eats using something called "Aristotle's Lantern." It's on the underneath side of the urchin, and it uses these 5 radial teeth-like plates to scrape rock of algae (the anus is on the top of the urchin). It will eat basically anything it comes acoss. I've never done anything to feed him. I've never seen him eat corals or macro-algae.
They are very similar to starfish and moves about the same speed, though poses little (or possibly no) threat to your tank. I bought him because I was assured it cleaned the sand. He will go across the sand, but no more than running across rock and the glass. I am pretty sure it does not deplete your pod count. Some urchins are dangerous, though this is not one of them.