Hitchhiker crab, good or bad?

[EugeneReef]

Can anyone identify this rather large crab that came on a piece of mushroom rock I bought recently? (Probably 3.5" across from leg to leg)

Video:
https://gfycat.com/SinfulNaturalBackswimmer

Photo:
https://imgur.com/a/2lOV2UV

Superficially he bears a resemblance to a Mithrax crab, but has a more oval body shape and the eyes are much further apart. It's hard to see in the photos and video but he does have the little white algae scraper things on his claws. He's very timid and only comes out when the lights are dim.

Due to his timid nature and the algae scrapers on his claws I have left him in the tank for now.

 

[AlSimmons]

Xanthid crab perhaps? The link below might help.

http://reefkeeping.com/issues/2003-12/rs/index.htm

 

[EugeneReef]

Xanthid crab perhaps? The link below might help.

http://reefkeeping.com/issues/2003-12/rs/index.htm

Thank you! Based on that rabbithole, I believe it closely resembles Chlorodiella Nigra. http://decapoda.free.fr/illustration.php?n=3&sp=163

Or at least some kind of Chlorodiella.

 

[Uncle99]

Beware of all crabs, they are opportunists, and somewhat less predictable in the confines of an aquarium. Most will take a swipe on slow movers if given the chance.

Whether just walking on corals making them contract to consuming snails or slow moving / sleeping fish, since they really don't remove a ton of algae, I steer clear of any.

I completely understand those who do keep crabs and support them, just since banning them in this latest rebuild, no losses in two years now, not even a snail.....hum?