I have 4 urchins all hitchhikers.
1 pincushion for 3 years and dont do a thing but does knock over frags all the time.
1 that I cant id and he is fine as well
I also have 2 urchins. They will and do eat polyps. Munch munch munch. They also eat algae. I pulled them from the main tank when i figured this out and moved them down to the refrigum. Where they began to desimate my cheato. Then i moved them to a rock only tank.