I've got a ton of this stuff on my glass, too. Although, I've been told that it may not be coralline. I'm sure it's the same stuff you have. It's just about as hard to scrape off as coralline and grows in a similar fashion, but at a much faster rate. I've left the back glass on my tank untouched and the purple coralline seems to just grow on top of this green stuff over time. HTH. Lance.