Yeah check your nitrates and phosphates. The gha might be absorbing it before you can test though. Nopox is Red sea chemical but you can vodka/vinegar dose as well. You can google how to lower phosphates and nitrates you are by far not the first salty to have this problem. I'm assuming you're using rodi water.
Here's what I would do personally
1 -limit food input and ensure any food being added is getting eaten
2- start doing large water changes frequently. I would do one 40% water change wait a day and repeat for 3 or 4 water changes to help reset parameters and weaken the existing hair algae.
3- remove as much hair algae by hand as possible, the water changes should help weaken the algae making it easier to remove.
4- implement gfo or other phosphate binding media slowly over time to help ensure that the phosphate isnt being absorbed by the alage first