I had one of these come with a piece of live rock that I got from an offshore reef one time in an old tank. It was the same shape, but a slightly different color (could’ve been older than yours). Anyway, I kept it in the tank for awhile to be a part of the clean up crew just to see what would happen but it ended up nipping about half the tentacles off my anemone and killing several of my snails. I took it to my LFS and they kept it in a quarantine tank to see if they could identify once it matured a bit. Not sure what the outcome was since I moved out of town. This was several years ago but I definitely learned my lesson with unidentifiable hitchhiking crabs! I would proceed with caution though and see if you can get a for sure ID to make sure that it’s reef safe for your tank.
