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Your description seems like an acro crab.
You can look on Live Aquaria too for pics
http://www.liveaquaria.com/product/prod_display.cfm?c=497+501+2914&pcatid=2914
If it has black tip claws, its probably a gorilla crab
It is hard to see but it looks like this
uh ohhhh.... I actually bought 2 acro crabs from the LFS yesterday and both were white with some hairs and had black tipped claws. I saw them hiding in some acro colonies. He assured me that they were reef safe and figured he was telling the truth because they had been in the same colonies for weeks and he could've easily gotten rid of them.
I bought one from absouthly fish was about $8 but I put it on a acro and it did not sta and it wonders around the tank