First off Welcome to the board! Second, I will bump you back to the top for others to see, as I dont know what it is. I hate to see a clear pic get lost in the threads.
I don't believe it looks much like chaetomorpha...could be I guess...looks like a branching sponge structure that has flourished in my 55 ... mine is a white tube like structure that began about two years ago as a very small structure and eventually grew very large along the backside of my LR...is it sort of rubbery textured and hollow? Sure looks like a sponge from the pic. Then again I may be way off on my guess...
...and I'd also like to welcome you to the forums.
thank you for all your replys Ive never had so much help on my reef tank untill I started using this forum.it is very fragile when i try to clean off the algi it falls appartI am triing to find a good book that might help