Can you help to identify?

[cygmark]

Ok so i bought some live rock awhile back and set up my tank, and so on, i found this little crab like the size of a pencil eraser and it was beige with bright orange stripes.

I put him in the tank during cycling and never see him again, fearing after that it could have died, i fed my new fish today and what do you know, he comes out except he's the size of a quarter round, boy's been eating.

So i would post a pick but he runs away when i get close to the tank.
Is it harmful to leave him in there of should i try to remove him?

Josh