Below from WWM site
I'm just new to the hobby, I live in Scotland, I have just set up my tank about 2 months only haveClean up crew of snails and hermits and a few soft corals , this guy obviously hitch hiked on my live rock, he doesn't move all day and only when it's pitch black .<Ah yes; looks to be a Xanthid (member of that family of crabs); not "very" safe... I personally would remove, or at least isolate (in a sump; an enclosed area...) away from your corals, fishes. May sample in time. Bob Fenner>