I got this algae on a frag plug, apparently I missed it when I added it to the tank. It seems very invasive and my starry blenny and tang will eat it, just not very fast. It also seems to die when my BTA lays over it and it is very brittle.
Looks like Caulerpa mexicana or one of the related species. It's highly invasive so pull out every little fragment you can see and hope the fish take care of what you can't see.