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. Urchins are known bulldozers, so keep an eye on your corals if you purchase one, as it may knock a few over. Reef-safe.