I have seen 2 of these in my tank since I set it up on May 1st. Most of the time I never see them, the couple of times I have they just hang out on a rock. During those times I've never seen them go near my corals or my coraline. They're usually near a hole so they can retreat quickly, too. I've been told they don't get much bigger than a nickel to maybe a quarter at most.