Wasn't an I'D really, there are 1000's of kinds of crab that look the same and stowaway on LR.
Mostly these crabs just get called reef crabs but there are s few things that I look to see to check if he's gonna cause grief.
No1 look at the claws, if they are massive and look Luke they are made for ripping fish apart there is a fair chance that is what they are in fact used for, if they are smaller and socketed (google mithrax crab claw) then the bulk of their diet will likely me algae.
No2 if it isn't a mithrax and it has hairy legs it may grow into a teddy bear crab - this isn't a nice thing.
No3 watch it like a hawk for a few days, try to feed it and see what it does cos at the end of the day one crab may be a killer whilst it's brother is a gentle old salty soul,
If your to worried just whip him out and take him to your LFS, they tend to deal with em in one off 3 ways
Sell if they can (people do buy them)
Sump if they can't sell
Feed them to puffers/mantis/trigs if they don't want to sump him.
The above is what I was told about by family of crabs, I've let them be and they seem to be behaving.