Stuff eating away my liverocks… and some feathery tree thing.

I’ve noticed one of my liverocks slowly being eaten away by something and I’m a little worried:
I’ve taken a couple of photos of it.. seems the rock goes all white then starts sinking in:

and

There’s a tiny mantis shrimp inside the rock too, so I’m not sure if he’s contributing to it or not. (I’m still undecided about the mantis shrimp – I wouldn’t know how to get rid of him anyway) – but what should I do to stop this green/white stuff from eating it all away?

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Also.. anyone know what this is that’s growing off my live rocks?
It looks nice.. so curious what it’s called and hoping that it’s beneficial – got a few growing off another live rock and they look like pretty little trees with featherduster style ends.

Appreciate all feedback and thoughts!

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