Crab ID?

[BozoTheClownfish]

Hi everyone,

I just noticed an unexpected new crab in my tank. It's a "normal" crab, as in "not a hermit". It's about the size of a quarter, body + legs. It's mainly black with some white banding around the joints of the legs. The claws are relatively large.

It always hides inside my rock... I've never seen it out in the open. Sorry, I can't get a picture; it's very elusive!

Any ideas? Is it dangerous?

Thanks!