Is this a sponge? Should I kill it?

I think this is a sponge, but am not sure. I like it. It's on a rock with a bivalve that is thriving (now that I've said that...) that has been alive for the past year +. The black thing is starting to spread. At this point I can dislodge the bivalve and keep it but toss the rock if the black "thing" is going to cause trouble.

Thanks for advice.

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