nah, some urchins will hurt corals, many wont though. Sometimes it depends on the urchine istelf and how much it wants camoflauge. or if it just goes over the rocks and your zoos happen to be in the way. Their teeth are like grinders and im sure youve seen by the trail of white rock it leaves.
it could definatly be the temp if this happened over the period of a few months
or even the cleaner shimp. some cleaners and pepermints have been known to attach the zoos, however most will not
i believe that sundial snails, fire worms, ect are a definate possibility. i have noticed that one of my patches of zoos seemed to be receeding. I had found a small snail that looked liek asundial a few months ago, but i squished it. every night i look in my tank to make sure everything is well and i dont have any hitchickers. in that time ive seen random crabs, a pistol shrimp, the sundial snail i was tlaking about, tons of different hitchickers. But as it turns out, the other night when i was looking, i found my CBS eating a softcoral eating bristlewrom. The sides were all furry and everything. I had never seen one in my tank, and i ahvent seen any bristleworms in at least a few months. So it jsut goes to show you that thigns can come up.