Some pics of your tank and your sump would be most helpful, using white light only.
Please give us an overview of your tank. How many fish of each type and their approximate sizes. What do you normally feed and dose in a day or a week (nutrient import)? How do you export/remove nutrients? GFO, macro-algae, chemipure, skimmer, GAC, etc. What lights to you have and how long do you run them?
In general, do you want to keep your tank completely free of even a spec of algae (since any algae is a sign of failure), or do you feel that a reef tank should have some algae in it (but algae shouldn't be the dominate occupant)? Have you ever added Live Rock (full of sponges and other micro-fauna), Miracle Mud, Garf's Grunge, or anything similar to your tank? How long ago did you use Fluconazole?
CUC won't eat GHA once it gets long, so you have to manually remove all you can first. I start a hose siphoning into a filter sock in my sump, then pinch the GHA between the end of the hose and my finger and pull it off. Down the hose it goes.
Once I've pulled out all I can, I blast what I can't reach with boiling tank water from a turkey baster. Stop all flow so it stays in contact longer. It'll turn bright green and die. You CUC will go crazy over it too. Even my clowns will pick at it.
You may need to do this a couple times per week for a while, but eventually your CUC should be able to keep it at bay. I also suggest a refugium and daily phyto dosing as has been suggested by others. I dose 4 oz daily and keep my refugium light on 24x7.
I did use reef cleaners recommendations to stock my tank and refilled here and there as they get diminished. Astera snails are out, I have turbos and trochus ceirth and nassarious, a fighting conch, brittle stars, dwarf blue and regular red hermits, short spine and pincushion urchins, lawnmower blenny, powder blue, sailfin, hippo, and bristle tooth tang, and a rabbit fish. The expectation being that once I pull or kill off the long gha, the cuc will keep the shorter stuff under control. I don’t mind some algae but it’s over taken the tank. I’ll get pics in a sec. My other fish include 5 or 6 clowns, an emperor angel, 2 anthias, 2 cardinals, a leopard wrasse, and 3 chromis.
I feed once a day or so. Black boxes ViParSpectra blues 11 hrs 100-%, whites 20-% 5 hrs. Live rock, live sand, protein skimmer. Dose alk and ca daily on pumps.
ok I’ll def up my phyto game, just ordered Tommy’s. Should I still do a red light scrubber in the fuge?