One way is to remove the rock that it's den is in, and try to extract it that way.
Or...Make a trap using the inverted pop bottle trick....
Or...there are some commercial traps available for purchase....UltraLife’s “The Trap”. (pg. 84 of Big Al’s product guide)....or "TrapPest" by AquaMedic.
Or...just leave it.
I have at least 8 crabs (probably more), of various kinds, in my 135g.
I tried catchin' em with that bottle trap, but I don't think they even knew it was there. In my humble opinion, and based on my newbie reefing observations, crabs don't seem to venture far from their den. The crabs I have watched in my tank pretty much just munch on coralline algae right outside there front door.
Maybe I'll run into problems some day, but so far, IME, they are not a problem at all. (It's also much easier to leave 'em alone. As long as they behave).
JME & O.
Cheers,