I added epsom salts for a while but didn't have a Mg test kit and didn't want to push it too high. Eventually it mostly turned brown/clear except for the holdfasts, looked like I was winning. Then I had to go out of town for a while, when I got back regular water changes diluted the Mg. Now the bryopsis is green again, though not growing very quickly. Got a Mg test kit, current Mg is 1520. I think I'll have to boost it to 1800ppm, I'll do it kind of slowly so as not to shock new livestock.
Also, when it starts to work keep an eye on your overflows, especially if you're using HOB overflows. The teeth of the surface skimmer of mine gets really clogged, raising the display tank water level.