The tank has lights from 2pm - 11pm (an hour of ramping on either end) LED and T5 (globes are a couple of months old) The T5's are only on for 4 hours per day.Whatever started it growing, there's something you are doing (or not doing) that's keeping it growing.
Either you're introducing too much nutrients (how much do you feed and what do you feed?) or the nutrient-removal system you use has failed.
How much light does the tank get?
Anyway, probably didn't have to say it as everyone would be thinking it.
I have had this same schedule for years now.
I think my Herbivores are just lazy and eat the food I put in to the tank. I feed one cube of frozen and a pinch of flakes one day, then the next they get a pinch of pellets and a pinch of flake.You could also get yourself more herbivores or feed less the ones you already have.
My phosphates measurements have been around 0.64 or higher and my nitrates 2 or less (often 0.5 or less), for the past two years or so. I don't have sps so no problem there to worry about. I don't use any chemical removal system except for a small skimmer (under-skimmed compared to everyone else here). But, I do keep one wall of my tank glass for algae to grow on and I keep an eye on how much of that algae is getting eaten by my Yellow Tang and Coral Beauty. The Yellow Tang is useless at voluntary algae control compared to the Coral Beauty or a bristletooth tang.
I reduce my feeding of Nori when my fish are not taking care of the algae.
I think the Yellow Tang especially is lazy and will allow itself to be content on Nori if given the chance and will leave cleaning the rocks and tank wall alone.
Previously I had a bristletooth tang and when I got rid of that I did get the GHA and it took a while to go. The way I got rid of it was to reduce feeding all food types, especially the Nori.
So, I would be looking at reducing your feedings and forcing your herbivores to do their job.
Nori is suppose to be high in phosphates and I feed a fair bit of it now, but even with the high phosphate and relatively low nitrates I have no GHA. I do have some fine cyano so everything is not perfect.
I have a large Foxface, 2 Blue Tangs, a Sailfin, Purple and Power Blue plus a Red Spot Blenny and none of them touch the algae on the rocks, only picking at the back glass:rolleyes. I also haven't fed any Nori since the algae but I only fed maybe one sheet per week anyway.