Killer underwater spider or ...?

[squibege]

Okay what the heck is this? This is the only live rock that has this- got it out of a fellow reefers tank. A scarlet hermit crab that was in the tank for 2-3 days crawled into the nook and now I'm pretty sir he is no more (hasn't moved a bit in the last 3days) When I blasted him with a turkey baster last night I wasn't sure if he moved a leg on his own or if it was just a power head. Also seems paler than the others.

Any ideas what could be living in there? Do I need to get this rock out of my tank?