Its a pretty easy thing to do, just takes several months to do it. First off, if you have any snails in your tank, you will have to remove them before you start all of this, they won't make it thru this alive, crabs will do alright, most of them anyway. Next up you want to buy a new Magnesium test kit, the reagent will be out of date at some point. You want to make sure you start off with a good test kit.
Ok, what you need to buy is ( Gerwin sells this stuff ) Kent's Tech M Magnesium, buy at least 3 bottles of the stuff, you may need a fourth, at some point, depends on the size and TWV of your tank. Any other magnesium won't do,,,, has to be this stuff ,,,, its not the Magnesium that kills the Bryopsis, its the trace elements in the Kent's that kills the Bryopsis. I guess its what they add to the Magnesium when they are making it.
You are going to start adding 100 ml per day to your tank, you are going to do this every day. When you make water for a water change, you are going to dose the new water you are making, up to the Magnesium level in your tank before you do the water change using your test kit. What we don't want is the Magnesium level in the tank dropping.
At some point the Magnesium level in your tank will get higher than what your test kit can measure, most test kits will only go to a 1500 or 1600 level, you are going above this. When you get to this point, you can still use the test kit, and here is how you do it (( I got the info on how to do this from Wes )). You take 1 ml of water from your tank, add 9 ml of ro water to it, this gives you 10 ml of water. Use your test kit and measure the Magnesium level in the water, take the number you get from the test and multiply that by 10. That will give you the Magnesium level in your tank.
I don't know what the Magnesium level in my tank got to during my battle with this stuff, I didn't know how to measure it at the time
I know it was at least in the 2000 range though, because I kept adding. I would get yours to the 2000 or 2100 level and maintain it at that point. Ok, what you are looking for in the Bryopsis. Bryopsis kinda looks like a Fern plant, a main stem and branches coming off of the main stem. What you are going to see first is those branches coming off of the main stem start to disappear and just leave the main stem. At some point the main stem will start to disappear. At this point you will think you have your problem cured, you don't. I would keep your magnesium levels up there for at least a month longer. No since taking a chance, when it has taken you this long to get to where you are at now, a bit more won't hurt.
I went thru this process twice in my battle
I thought I had it licked, but didn't. I mount a lot of my corals in my tank ( mostly SPS corals ) in my tank on the frag plugs that they came on. When I thought I had this battle won, a few months later I saw the Bryopsis start to pop out from underneath these frag plugs. That is why I say, when you think its gone, keep those levels up there for a extra month or so. I have a lot of SPS corals in my tank, I didn't lose any during this process, but I did lose a lot of growth that the corals should have gotten during this, growth really slowed down. I have read you can get Magnesium levels to high and start to lose corals. While I was going thru this, I was watching corals more than I was watching the Bryopsis, so that I could stop dosing id I saw my corals were in jepority. Hope this helps you