What is this thing?

[Roggio]

I spotted some kind a black crab in my rocks. I pulled the whole rock out and cannot get him. He’s black and really large. I have no idea where he came from. While I had the rock out I found this thing. It has a hard outer shell and moves like a snail but really fast

 

[Roggio]

Update, my eel ended up going after the crab with no luck