It did cover a 1" gap between the rock and the back glass. You can see it in the picture. I'm excited to see that because it will look awesome if it covers the back.
Mine does this too. It's just looking for new places to spread. I limit it to one rock pile in my 150G so it doesn't take over the entire tank. When I do a water change, it's easy to just grab and break off any that are heading somewhere I don't want it to go. Mine will grow up the back glass as well, but again I don't want it spreading to other rock piles so I periodically peel that off as well.