I have a pencil and a purple urchin for about two years now. I have corals as well in the tank and have only found the pencil urchin eating a coral once just the other day. The pencil urchin attached itself to the base of my devil's hand leather and the leather was not happy. I pulled him off and the leather recovered the next day. Other than that, I notice the urchins will normally eat around the corals when moving rock-to-rock. I hope it remains that way.