It looks like your garden-variety brittle star ... ?
Or, I should say, it looks just like the thousands upon thousands of the little ones that I have in my setups. They don't seem to harm anything (the ones that I have) except that they will seek out any nook and cranny and that does include the SPS. You should see all the little arms that come out of nowhere out my M.cap when I feed the tank (not to mention all of the rockwork, and the powerheads, and etc.) In fact I seem to regularly have to pry them out of the pH probe, which is about the only thing about them I find kind of annoying.
I thought everyone had these.
No? Mine just hitch-hiked in on rock ...
Or are my eyes just deceiving me and this one is different from those?