You have a
Hairy crab (Pilumnus sp.)

or a Mithrax

Crabs that are used for algae control, may be fine for awhile, but with all crabs being omnivores,
it is usualy only a matter of time before these crabs discover that there are meatier items to be had within your aquarium.
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125gal.Fish Only (Born on 11/25/06)
75gal. Reef Tank (5 yr old)
20gal.
Tiny's new home
30gal. Extra high FW angel tank ( 16yr old )I traded for this
5gal. Food tank