Tank is established several years . Water params are good . T5 lights bulbs 2 months old .Have other tanks no problems. Tried peroxide dosing , raised mg ,and algae fix. At this time a few coral ,1 fish , hermit crabs an snails.
I think you can get it under control by adding a few steps. Get a large cleanup crew of snails and crabs. Run some phosphate remover, cut back on your feeding regular water changes. My tank cleaners made short work of my problem and I haven't seen hair algae in a long time.
Bio Pellets, Prodibio Bio Clean or both. Plus some Mexican Turbo snails.
I don't think a phosphate reactor will help for this situation.
PHO4 minus is also a good option.
SAT Marine will also help lossen the algae so you can siphon it out easier.
Check water parameters especially alkalinity. Make sure that you do not have bryopsis.
If you have bryopsis - remove your LSP corals and raise mag to 1500 using with Kent or Brightwell - other brands will not work - till it is all gone.
Test your RO water with a TDS meter.
Good luck.