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. Plucking it by hand is possible, but some bits will be left behind. Be sure to use a turkey baster 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 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 couple of days.