I delt with a nasty out brake of bryopsis several years ago that overtook much of my tank. I had gone way too long between water changes. Life sometimes gets in the way of the tank.
I removed as much as possible with a tooth brush in a tub of water. For the rock in the tank that couldnt be moved, I again cleaned the rock with a tooth brush and removed the algae from the tank with a small fish net.
I found the key to controling this algae, at least in my tank, was removing the phospates. I started with using phosGuard in a HOT wisper and am now using phosban in a reactor. Within a week of adding the phosphate memoving material, the algae lost its dark green color and started to die off. It took a good six months, but I did get rid of the algae without braking down the tank.