What kind algae is it?

[reef-geek]

tank is over 3 months old, rock is covered in green, not sure what kinds alage is it, it's not getting worse or better for the past month or so.

 

[BrianAnthony]

I am no expert.... If this was on my rock, I would write this stuff off as bio-film on the rocks, which is a fancy way of saying it's beneficial. I start to worry when the green stuff starts producing oxygen bubbles. This means to me that the algae is in full growth mode. My rock started like yours at around the 3 mo mark. After a while though, I had green hair algae. Definitely, be in algae removal mode if you see hair like algae filaments. Hopefully, others chime in to give their opinion.