manually removing some of the hair algae. Next time you do a water change, attach a scraper to the vinyl hose and scrape off the hair algae on your glass while your siphoning it out
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If most of it is on your rocks, you can always take your rocks out with some of the tank water, then in a 5 gal bucket scrub the rocks with a toothbrush in all of the hard spots, then rinse with freshwater (ro/di) and immediately dip it back into saltwater. The freshwater will rinse off all of the spores.
Something less drastic that you can do is cover the tank for 48 to 72 hours, and after it is dead, immediately do a water change and suck up as much as you can (no more then 50% water change. Then add a phosphate sponge. Make sure to remove to sponge after a couple of days.
You may be adding phosphate to the water by feeding too much, using a bad source of water etc. Change up some stuff and you will have better success.
Good luck!