What is this, it looks like a plant


Reefing newb

I have like 3 that came on my live rock when I got it 3 months ago. They don't seem to grow but they are green like algae, and look like clam things, and I'm not talking about the hair algae, I'm talking about the thing that has hair algae on it. If you try to yank on them they are fixed to the rock.
No they can't be, they are soft and bendable like a plant, it's just they are rooted in well. They are dark dark green like a withered plant, and the inside is little layers of plant like material.
David is actually right:D
That is a type of mussel that anchors itself to the rock.They dont last long in our tanks because they just cant enough to eat in an enclosed system.