It could very well be Green Coraline. The more types of Coralline (green, pink, white, purple, red) that you add to your tank, the more you will see growing on your Live Rock, substrate and aquarium walls.
See this:
I hope they say it is coraline because I have noticed the exact same thing growing in my tank. I have also started to notice a dark dark red starting on the back glass and then a little on the front hopefully it is coraline as well. I ran my magnet cleaner over it and it took part of it off but not all.
i had an all white rock. i mean white. and then as it started to get coraline i noticed some green. only one rock in the whole tank has it right now. the rest have eithere purple or nothing. i will try and put a pick up tonight