Your Yellow tang is a lazy bum. Rent a sailfin.
Seriously though, the biggest problem I see is that you have a foam/rock wall, so you can't just remove the infested rock. It's not a species I've ever seen before either. It looks soft enough that your YT should at least nibble on it.
I really hate suggesting buying an animal to control something and then it turns into a pest of it's own variety, but again since those rocks are fixed, you may not have much of a choice. I've had macros get loose in my tanks also, and if the tangs don't do it in, I whip out the tweezers and pluck away. It's not fun, and seriously tedious, but it is an option. The thing that will most likely mow it down in a hurry is a sally lightfoot. They really are green-stuff hoovers. But once they finish, they'll turn on snails and othe inverts, so you'll have to trap him once it's done.
It wouldn't hurt to cross-post this in the macroalgae forum and see if you can get an ID, and some suggestions there.