I have 2 pencil urchins, they are both bulldozers!
I have to knock them off of the rocks all the time when they are headed towards my small corals, I don't want to chance them getting eaten. They have knocked rocks over, pushed the heaters off of the suction cups.
Plus they eat coraline, they scour the rocks clean when they find it. I keep thinking of getting rid of one of both, but they are still cool to watch.
I have even fed them, take a strip of nori and slip it under the spines when they are on the front glass, the urchin will pull in the nori and eat it! They are both larger then a golf ball!