unidentified mystery crab!

[365seasons]

My husband and I were doing a water test on our saltwater tonight, and out of the corner of my eye I saw little pinchers coming out of one of the holes in our liverock!

Upon further investigation we found a dark purple ( ? ) crab about the size of a toonie.

We found a couple little tiny crabs dead () when we first set up our tank, and just figured they came in our rock, but this guy must have been living there for 3 or 4 months at least now!

Never seen him before, and as much as he was poking his pinchers out, he didn't come out. We were pretty excited.


Is this normal for liverock? Any ideas on what it may be??
We can try and snag some pictures if he ever comes out further!

Thanks in advance!