I would raise your mag by 100ppm like psu4me said with kent tech m magnesium. It worked for me a few years ago. I actually documented it on here or on reef central. I will try and find it. Unfortunately it came back and is now immune to the tech m (I've been told) because it wont DIE!!! Keep it at 1600 for awhile until it is all gone. Took about 3 months total for me to raise the mag, kill the bryopsis, and get my mag back to normal level. Also pull out as much as you can manually, I found that to be a major help. Can u take a pic so we can see how much you are dealing with?
Also, back on track, I didn't loose anything in the whole ordeal.
If you have the room I heard a fox face will munch on it. I'm going to try one and I'm going to try some lettuce nudibranches as well. Worth a shot I think. I hate this stuff!!