Other people think this may be Dinos.
If they are dinos, you should be able to completely remove them using an airline tubing siphon, which is a section of hard airline tubing like what is used in an undergravel filter in the towers, connected to soft tubing. You should be able to siphon nearly 100% of them out and only take out a gallon or two of water (in like 30 minutes, it takes time!)
If you can't siphon them off like that then it's not dinos, it's hair algae (which attaches firmly)
Dinos usually pop up when you make major changes to flow or rock structure and usually they will burn themselves out after a few weeks as long as you try to mitigate their growth. It's my opinion that there is no one solution to getting rid of them, you just do it all and manually remove and let them run their course. This is why you hear "XYZ worked for me in 2 weeks" when all they really did was mitigate and time took over...JMO like I said...
You could always manually remove it with a toothbrush
Scrubbing the rocks off is one way to temporarily remove the algae, but it also can remove the periphyton which is where a lot of the bacteria live, so you in essence "reset" that and then your tank (rocks) might go through some mini cycling as these bacteria re-establish (not a nitrogen cycle, a bacterial growth/death cycle, which is more invisible to our test kits)
If you were to put an Algae Scrubber on the tank, it's going to have to compete with the tank algae, which is in effect, an algae scrubber. So you may have to do the manual removal thing to some extent just to get the scrubber ramped up and growing. But maybe not - every tank is different. So a scrubber may work here, but it's going to take time because you've got something else going on here that it's going to have to compete with or battle against - there is a nutrient source in the rock.
the source though is the thing to focus on. Since it's on all your rocks, it could be that your rocks have become saturated over time due to high N and P conditons, and now that they can't act as a nutrient sink anymore, the algae grows. If that's the case, chances are if you got super aggressive with N and P removal, the algae would continue to grow for a while (months). There are a couple ways to fix this potentially. The last-resort is to nuke the rock with a Muriatic acid bath, which kills everything and dissolves off any calcium-bound phosphate (along with the rock itself). Another is to cook the rock like it was new, and be aggressive on skimming and GFO in the curing tub and let bacteria do the work; this takes longer but your rock won't have to cycle again.
I would also consider your light schedule, I'm not the expert but 8 hours of MH to me seems a bit much. I might go to 6 hours of MH peak.
I would like to get the Po4's down to the least point of .02 ppm and the No3's down to .04 ppm and I will be happy. You can run some kind of carbon, I am running biopettlets in a small reactor and GFO and my GHA is almost gone. I am now dealing with red slim and I am going to change my RO/DI filters tomorrow and this should resolve my issue in another two weeks. 
