Depending on a size of you tank try several different sea urchins.
Tuxedo and Lava is good choice. Lava Urchin eat my clove polyps (large green type) too that's why i sold it. But that may be a benefit for someone else.
Po4 is there even you are testing zero or low p04 that's in water. Your rocks are "Home Depot" of phosphates and that what HA is feeding on.

Fish will not go and eat hard hair algae if other softer food is available (pellets).

If you don't have light sensitive corals lower or turn off you light.
Also you can cover HA with shells.
Well regular water changes as well as other techniques to eliminate phosphates and other sources of HA foods will obviously work, but it is harder now when HA is there.

Well the only thing that comes to mind is reduce the light cycle per day, emerald crabs and huge snails would work on it in the mean time.

If it's limited to that area & not impacting any of your livestock, might as well rejoice, you have a free algae scrubber. Best phosphate removal system there is.

Looks like a$$ though. I know it sucks po4 just not pleasing and it gets stuck to my power heads and overflow box.

My buddy had an emerald. No help with anything.