You might have more going on than a green hair algae growth. The overall brown appearance of the rocks and some wispy white growth (possibly bacteria) looks like what happens when I purposely nitrogen starve a brightly lit aquarium. The poor coralline growth is noteworthy for an aquarium of this age.
I am sorry that I don’t have suggestions how to fix your aquarium (I usually empty my experimental aquaria when they reach this stage). I think getting the periphyton (growth on the surfaces) looking like a typical reef aquarium may take awhile and there will likely be no one magic bullet like buy more snails or just dose X that fixes it.