What the hell is this, algae sponge combo?

I have three of these growing in my monofilament netting barrier as the water leaves my display tank!
The rubbery bulb is on the out side of the mesh and partly in it and the thick and wiry coralline algae on the tank side.
This is like an individual plant growth with a bulb as its attachment and not something growing around something else.
The exposed to light part of it, is green and the underside has lost its pigment but is certainly not suffering.
The algae part flexes a great deal before it breaks like a brittle tree branch but it takes a lot to break it almost like plastic wire that bends and bends until it breaks.
I do not use live rock, all base rock was sterilised with hot water and bleach before adding and everything else is checked carefully before adding to the tank.
These are weird!