You could even run the Mg as hi as 1600 ... I'm doing this today, as a matter of fact
Please read the whole thread first as there are other ideas, and other stuff to know too, including how fast to raise the Mg
From Myka's guide
Get rid of Hair Algae and other Nuisance Algae
... Ok, you've tried everything...it's been a few months, and you're still brush cutting that HA. Now it's time for the big guns. If you have the Bryopsis type of HA (it's feathery unlike Derbesia which is grassy or like a green cotton ball), then this will work. If you have Derbesia, this won't work. Get yourself some Kent brand liquid magnesium. The Kent brand works best. It's cheap, don't worry.
Pump up your magnesium to 1600 ppm. ...