Perhaps the problem is your live rock.
Awhile back I bought someone elses system and merged it with mine. Since then I've had hair algae only on the live rock from his system. His rock is leaching phosphates, probably because he used nothing but tap water for his system.
You can control or slow it down with phosban, feeding, water quality etc...but ultimately it will only go away if you remove the rock altogether or once the phosphates have exhausted themselves from the rock.
I could be wrong but...
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