Bryopsis
if thats the only place it is growing, pull that frag and use a razor blade to help pluck the frag off of it and throw the rock away.
You don't want that algae.
Tubro Snails, Rabbitfish, Naso Tangs and Gratilla Urchins will sometimes eat it, but don't bet on it. IME they more readily eat it if it is weakened (Tech-M treatment). Lettuce Nudibranches will eat Bryopsis and only Bryopsis, but it takes a lot of them (as they are small) and in addition they are delicate as well as poisonous.