Is it coraline algae? Most people find coraline a postive thing, it covers the rocks and the back glass for a colorful backdrop. It can be a bit of a pain on the front glass, but regular cleaning will keep it at bay.
Yeah, it looks like coraline algae to me...
Which is a a really good thing...
But if you don't want the back of your tank covered with coraline algae, I guess you'll just have to scrape the crap out of it or use a razor...
But I'm not sure if that's what it is...
