monster crab

couldnt post in ID this because i couldnt get a pic of it to fast and stealthy after a water change i was looking at my tank and saw this giant black claw reaching up an appearing to be picking at one of my nicer acro colonies closer look it seems to be a monster shiny black w/possible white band around claw.the body on this thing was at least the size of a quater if not bigger i tryed to snag it but no chance anyone have any idea if it would be attacking my acros? i have pulled many gorilla crabs out but never anything like thisi probably know this is not a good thing but i really dont know how to get this out gorillas were easy this guy moves any ideas would be appreciated

 

You can try a trap. The easiest way to tell if it will be harmful is by the shape of its claw. Pointy claw means trouble.

 

you think that is a monster crab, have a look at my girl: Stonie, she is a pound stone crab that rolls around volleyball sized 15 pound rocks like nothing. But like Vinny said, the claws are what tells you if you need to get this thing out, and usually they bad ones have black tips on their claws. . . (Stonie is one of these types of crabs) Also, if possible try to catch him out in the open, then put a big chunk of shrimp meat in his face, when he grabs it usually they will hang on long enough for you to yank them out of their holes, but be ready to catch him if he lets go.

BTW that rock behind her on the right is the size of a really big grapefruit

 

You can catch them a lot of times by putting a meaty (non-floating) food into a glass that you lean against the rock. It needs to be made of glass so that it's smooth. (They go in to get the food and then they can't get traction to get back out).

I've kept some really interesting stone crabs in species tanks before. They are quite destructive in a reef tank but fascinating in their own tank.