Well, I first noticed some of this yesterday, I tested a couple of water parameters which didn't seem too bad. 2->3 ppm Nitrates, with 0.04 ppm Phosphate. There seems to be even more spots of this algae now on the rocks.
Some quick googling seems to suggest it might be a precursor to coralline, cyanobacteria, hair algae, or green slime algae. Anyone recognize this from the photo?