Sea urchins are mostly eaten by triggers, eels and puffers/boxfish. As long as you don't have one of these they won't be eaten
. They only eat all forms of algae from short to long haired -.^. They won't touch your corals, but if they are not secured to the rocks they will be knocked down. The long-spinned sea urchins are poisons, but should be safe for humans. I would highly recommended getting a sea urchin that has short spins. They EAT a tone of algae a day. So, if you don't have much algae to go around don't get one. If you have an algae problem they are your cure.