Could be green coraline algae
Coralline comes in a range of colors, the pink or purple variety that we generally keep is the easiest to propagate and most common as far as aquarium life is concerned. Red, green, purple and blue are some of the vast colors of the coraline rainbow.
Its VERY hard to get pics of. It just doesnt really show up in pics. If you look hard you should be able to see it. I see it in the pics.