Good question, thanks for asking!
It was a long road.
Battling any algae outbreak is always extremely difficult. There are so many variables. And then we want it gone as quickly as possible so we tend to be impatient and implement multiple changes so it's hard to know which individual thing made the difference, or which combo. But here's the strategy I implemented, pretty much listed by priority.
1. Nutrient control (might not be what you think)
2. CUC
3. Manual Removal
4. Water Changes
5. Reef Moonshiners - is anything fueling it?
6. PATIENCE!
Notice that there is no bottled "quick fix" up there.
I strongly believe in finding and fixing the source of the problem, not using a bandaid sort of solution, no matter how many people say it worked for them. I knew there was SOMETHING that caused the HA to appear in the first place, just needed to figure out what it was.
And past experience told me it wasn't lighting. So I didn't even mess with blackouts, reductions, spectrum changes, etc.
NUTRIENT CONTROL: I had fluctuating nutrient levels. Started with high p04 and no3, then it dropped to nothing, then it was back up, then back down to near nothing... Never at zero for long but it hit undetectable levels. One thing I have learned over the years is nutrients are not a bad thing but ZERO nutrients are and can lead to the dreaded dinos. So I concentrated on stabilizing Po4 and No3. No3 in the 5-6ppm range, knowing I could deviate a little either way. And p04 between .06 to .1
I had to dose both of these, putting them on my doser.
CUC: Tried to up my CUC collection. More snails, hermits, etc. Mexican Turbos seemed to help. I tried a couple of back short spine urchins, but they barely moved around for me. Tried a couple of emerald crabs, neither of which survived. The best thing by a MILE was the Sea Hare. He mowed it all down. Thought it was gone and didn't want him to starve so I brought him back to my LFS. Unfortunately, it came back!
MANUAL REMOVAL: Absolutely HAVE to do this. I did this pretty much every weekend. Used my fingers to grab what I could, then went back with a stiff toothbrush and scrubbed off whatever else I could manage to reach. Most CUC (other than the sea hare) won't touch the longer stuff. It has to be right down to the rock.
WATER CHANGES: I wasn't doing them really, since I do the Reef Moonshiners which replaces all elements. My socks and skimmer were doing the nutrient removal. But I went back to doing small daily water changes. Easy to swap out 4 liters of water on a small tank. The small changes wouldn't mess with the moonshiners program.
REEF MOONSHINERS: As I said, I follow this program which replaces all elements. ICP tests weren't showing anything either way. Nothing really elevated. But I started thinking about which minor ones could possibly be contributing to the algae, or at the least preventing it from going away. I have no idea if this helped, but a few weeks before it really stopped coming back I stopped my daily IRON dosing. Could totally be a coincidence and just the fact that all the previous items above had been working for me, but it was at this point I really starting seeing a positive change where I could definitely say it was on it's way out. When I would get to my weekend algae-pulling chore, there were be noticeably less than the previous week.
And here I am... totally free of GHA!
Can't really say for sure what it was in the end but I'm a happy camper.