What to do?

[George]

Your bio-load seems to be low for the tank. So the nutrients that feed the algae must come from somewhere else.
Do you use ro/di water for water change? what skimmer if any? is it second hand LR (may leak PO4)? you are not crazy at feeding the fish are you?
let's treat the cause not the symptom.

[pinkreef]

that tank looks beautiful to me! my 2 tangs would eat that up in a week
maybe you could trade rocks with someone who needs that macroalgae.

i buy it for mine.

[BlueWorldAquatic]

pick out as much as you can, the macro algaes are perfict for a friend that has a tang.

then borrow a seahare, he'd clean off the roots and viola, no more algae.