always wonder about this stuff. I'll have to do more research on this.Use Flux RX and it will definitely fix it.
Flux Rx (Fluconazole) Aquarium Treatment
Flux Rx is an extremely safe and effective aquarium treatment and can even be used in reef aquariums with corals present. It should be noted that if you have bryopsis or green hair algae present in your aquarium, fluconazole can cause rapid death of these algae species as a side effect. Hair...www.bulkreefsupply.com
I used it 2-3 years back (and added Zebrasoma to keep algae proactively in check) .... 0 GHA since.
I am going to still ad an ATS.
OP: if you do this way let us know how it goes or what ever you do.
I know there are many many ways do deal with GHA