Plant parasite

I need an identification.  I noticed during pruning my 10gal danio tank 
that I seem to have some sort of plant parasite on my green hygro.  The 
older leaves have a dark green substance growing on them.  The growth is 
not a slime; it forms strands about the size of fishing line (pretty 
coarse).  It seems to be consuming the older leaves.  I think it looks 
sort of like a version of Spanish moss (I bet Paul K. at Tugaloo knows 
this reference).

What is this stuff?  I've had the filament algae, green spot algae, and 
blue-green algae, and it's not any of these.