This long spined urchin is a Diadema Urchin. I recently purchased this for my reef because I find them to be beautiful. They are known to help mow down green hair algae, but urchins often consume coralline algae as well. This one is quite small, as you can tell by the neighboring zoanthids on the left. The yellow thing in the center is not an eye, even though it does seem to be looking around. It actually is the butt, and you'll see bits of what looks like white sand fall from there as it processes food. Urchins are known bulldozers, so keep an eye on your corals if you purchase one, as it may knock a few over. Reef-safe.
Don't get stabbed in the knuckle when working in your tank. It hurts, and the spine make break off under your skin. Also if you have an acrylic tank, their jaws may carve bite marks in the walls of your tank. This is another reason to keep your viewing panes clean so the urchin will look to the rockwork for food instead of scratching the tank walls.