Please ID my not so little stow away.

Okay so on Saturday I got all my live rock and arranged it nicely in my tank.
I was sitting watching it in the evening as alsorts of things were becoming visible already, when all of a sudden a big sandy beige hearly leg popped out of a hole in one of the big pieces off rock, I then watched as three more legs poked their way out before retreating back inside. this hapened several times thoughout the evening, and I wondered if I had positioned the rock so the main entrance was blocked and the critter was stuck inside. However it appers I had nothing to worry bout their as tonight he has actually broken away a lump of the rock and has emerged from underneath. he is very shy so I turned out the lights, after about a ten minute wait he came most of the way out. he looks like a sort of crab, but it dosent look like he has a shell, it also dosent look like he has any pincers either, and his legs are over 2" long and covered in spiney hair, each about the width of a toddlers finger. this thing is pretty huge and moves like a spider, I think he has clocked us watching him and has moved back under the rock and has moved a broken of piece of rock in front of him. What the hell is this thing and is it bad?!?!?!

Okay, update, I got a proper look last night. he is definatly some sort off crab, he has got claws, they are little shiney black ones. the rest of his body is brown and furry and he appears to have no shell. Can someone please ID this for me andlet me know if he is going to be a problem and if he should somehow be removed?

Thanks Orion girl, I now have a possitive ID of a Teddy Bear Crab. He is very sweet and I would like to keep him, they are apparently peacefull crabs, but I obviously dont want to risk a free stow away, attacking expensive corals or fish. Its very hard to find any info on him. I dont know if anyone here has any experience of them, and weather I should get rid?

Okay well I think I will keep him, Im just scared if he tries to eat hearmits or shrimp as they will cost me money!!!

He is sweet though, and he has fortified a little house for himself by blocking off entrances to one of the big lumps of rock by wedging smaller broken bits into the holes!