Unfortunately I do not know what it is, but it is very interesting looking that is for sure. There was a thread on here where someone posted pictures of all types of different algaes. He had a poll for you to select your favorite macro algae, but I could not find it with a quick search.
Hopefully the picture will make it easier for someone to help ID it for you.
I may be a little different than many, but I actually like the looks of it and would be willing to let it grow for a while. Hopefully it is something like Geoff stated and will not cause any real harm to the system. Personally that is a chance I would be willing to take, but that is also one of the reasons I currently have 3 tanks set up and quite a few empty ones on hand. :doh: