Wow looks like a cool serpent star but I dont know for sure I am suprised you got such a large specimen on live rock the LFS usually looks for the larger hitchhikers.
I would keep it if I were you. I think the harmfull ones have uneven leg sizes and shapes
also I have quite a few small slug like things that came in on an anemone rock,I cant get a photo as they are so small,only a couple of mm ,the larget is approx. 8mm,they appear flat like little flakes ,shaped like a figure 8 and generally dont move much although i have seen a couple wondering over the glass.
Thanks for the offer but i guess I'll just keep an eye on him and keep looking for an I.D..By the way where do you guys go other than here for photo I.D.'s etc.?