Well, if you can grab the rock it's on and pull it out that would be the simplest approach. If not, you can try a trap of some sort. I doubt you have a trap at home, so you could try a glass jar with some bait in it that's tilted so it cannot escape. If that doesn't work there are more effective traps you can buy.
Well it was a gorilla crab. When I went to get out, it floated up. Didn’t survive the cycling period. One less worry i guess. Poor lil guy. I’m hopefully in the last stage of the cycle, everything but the Nitrates are at zero.