ahhhhhh! need to catch a huge crab?

[Salinity Now]

It's a gorilla crab It wasn't doing any damage YET, but just give it some time. I'd notice it late in the evenings eating coraline algae most of the time. You deffinately want him out of the tank before you add any livestock. Hope that helps.
http://www.tbsaltwater.com/thepackage/watchout.html