That rock <"looks lovely">.The little green tufts have appeared recently and in my opinion looks very interesting to say the least.
Personally I look on green colour as a plus point, I think the rocks look much more natural with a patina of biofilm. You won't find any submerged rocks in nature that aren't colonised by plants or animals, unless they are continually scoured by sand etc.
It isn't quite the same, but what would the <"Statue of Liberty"> look like if they kept on scraping off the copper oxide so she wasn't green?
It is quite dark, but I think it looks like a <"Green Algae">, "Red Algae" (like BBA) can be all sorts of colours, but they are never grass green.Could the tufts be some type of BBA?
.......... and that would account for both the dark green colour, and lack of grazing?Close up it looks like Cyanobacteria matting,