I have a theory about the hair algae problem. Both, hair algae and cyanos prefer fresh substrates rich in nutrients (fact, not theory). When for example corals are dying, I think more often cyano and hair algae are the result and not the cause of the problems. I think this is the case here too.
The green coralline algae you have shown some weeks ago are no different species but it is a patch of dead coralline algae. Some of the pigments break down faster and this made it appear dark green. Also the brown batches on your rocks are dead coralline algae that may have died recently. It may or may not be a consequence of the antibiotic treatment. Coralline algae live, like many other algae, in symbiosis with bacteria and archaea for vitamins etc.. Also, at least in their pigments, red algae are very similar to cyanobacteria. Maybe they share other biochemical characteristics also.
The dead coralline algae are fresh substrate enriched and very rich in nutrients. Coralline algae are rich in nutrients "by nature". The GHA are just making use of these nutrients and are the first colonizers. It is a partial "initial phase". After the GHA have used up the nutrients they most likely would disappear like at the end of an initial phase.
This is in my eyes the best explanation I can give about the GHA problem.
