That's a preist hat urchin. Also known as Tuxedo urchin. Most places say that it is reef safe with caution. They usually are herbivores, and feed on algae and coralline, but if the food they normally eat runs out, they could move on to meatier foods, such as soft tissue of your coral. So beware.
I would always keep some nori seaweed stocked for that urchin if algae in your tank runs out.