What’s this and is it alive?

[shooter2377]

So got some live rock, and there is a couple of these things on it. They are pretty hard and a little slimey, and black/ really dark grey. Any ideas?

 

[Doctorgori]

the “slime” hints at something more biological than mineral…
I don’t like the way that thing is smiling at us, I’ve killed for less, but that’s me