A nuisance for sure, Bryopsis is a weed that has frustrated many hobbyists. Eradicating this type of algae is difficult because it anchors itself well to the rockwork. Very coarse in nature, this plant attracts any type of detritus which it then uses to create its own nutrient base substrate. Looking very closely at this plant, you'll note it has a feathery look to it which is how to identify it properly. You can remove some and place it in a small bowl of water, or spread it out on a white paper towel to get a better look under white lighting. Plucking it by hand is possible, but some bits will be left behind. Be sure to use a turkey baster or a small powerhead to blow out any accumulations to help starve this plant of nutrients. Snails, Sea Hares, and Lettuce Nudibranchs are all known to help decimate bryopsis. (This is not the same as Green Hair Algae - derbasia - which is much finer in texture.) Raising Magnesium with Kent's Tech M (and only this specific branded product) to 1600 ppm or higher seems to help eliminate this pest. Dipping affected rock or frag plugs to submerge only the bryopsis in 3% peroxide will turn it white, and it will flake off over the next few days.