Sounds like a Diadema. I currently have two of these that I've added in the past year. One was initially the size of a baseball or a little smaller, the other about a golf ball. In about 9 months they've grown to one around 8" diameter and the other to about 3". These of course are taking their spines into account. They've helped me control/ rid of a problem algae and have been great. Only occasionally will they knock something over, but never have devoured any thing but algae.
Mine aren't as bad of bulldozers as say turbo snails or my conch. The conch was responsible for pushing a blasto frag into my Baby's Breath favia and an acan. Not sure which, but they won, blasto DOA.
