Looks like good ole Caulerpa Prolifera. I have the stuff in my Seahorse tank. It grows quite rapidly. So if you don't have it in there for looks like I do, then I would remove it as it can take over.
Oh come on now. I have used this as an in tank food for angels and tangs for decades. It's only problem is the rapid growing rate which makes it ideal in a fuge. Pull some out as it grows as a nutrient export if you like the looks. If you don't, sell it to a LFS, they also love it.