You need a green sea hare. Not the pretty black spotted ones, but one of the brownish/green ones. They will mow down hair algae and clean a tank in less than a week. Then you need to figure out what is causing your high phosphates in your tank that is feeding the hair algae, and get rid of it.
Sometimes you need to give the hair algae a haircut first. I don't think even a sea hare will be able to eat 3" long algae. Cut it down to like 1/2" length using scissors and throw away what you cut off.
Many shops will sell you the sea hare for $30, then allow you to return it for $15-$20 so someone else can "rent" it. In this way they stay alive in captivity and don't die once the hair algae food source runs out. Some people have claimed to have success feeding them sheet Nori algae (Sushi), but I have never had one eat it.