It is the dreaded "Dragon Plant"
(or what Paintbug said, Halymenia
)
I saved a clump of it when I threw out all my valonia infested LR by digging the LR out around it and fragging out its holdfast. It is doing great after 2 months being the lone algae. I have pruned it 3 times already and I see a leaf tickling some frags, so #4 will happen later today.
Mine has done stellar in a strong steady return current to the main ph and it likes getting tickled by the jet stream of the main ph blast. the whole plant is constantly in motion. It gets the same light as the rest of the tank: 14hrs actinic VHO, 5hrs strong 10kK MH.
I am not sure how to prop it. maybe if I get bored, down the road, I will try to bisect the holdfast into 2 pieces, but I would risk f'ing up the whole shebang
ps: those little green BB's on the left: those are valonias and I highly highly recommend that you religiously hunt down and remove every last one of them, while you still can.......