I just won my battle with the devil-bryopsis, using the Kent Tech M magnesium method!
my tank had it really bad; and if you leave it untreated, it will take over your tank in a matter of days; at least, that has been my experience! Trust me, I read and I did alot of research on it...
*First of all, make sure you have the correct algae identified.*
It has a fern like appearance, and a dark green color...
*Second, are you feeding too much?? phosphates will only fuel it even more so!
*how old are your light bulbs? as your light bulbs get older, they change their light spectrum and shift onto the red spectrum, thus, giving growth to algae! your eye might not notice it, but any algae will!
of course there's more methods, but this is what I did, at first...
*bought a 64 once of Kent tech M mag..I have a 30 gallon tank..and used about 15 onces of it, but not at once!
*bought a Salifeart Mg test kit...not cheap $$, but worth having, imo
*lastly, changed my PC light bulbs to new ones.
First, I tested my Mag, and it was at 1250, a little low, but not too bad.
Then, I dosed it every day, for two weeks, no more than 50ppm...you can dose as high as 100ppm, but I only did it once. Keep in mind, I logged every thing!
Here, what you want to do is, IF, your mag is low, raise it UNTIL, you have reached 1500 OR 1600ppm.
Then, have it like this, for about two weeks. Once you have reached 1500 or 1600ppm, have the mag lower back to normal readings of 1350 or 1450 ppm, thru via regular water changes! During this process, if you decide to do it, you may, manually remove it OR have it come off by it self! But, imo, is better if you remove by hand!! is faster and easier.
this process, may take longer, depending on how bad your tank is infested...with this said, also, keep your eye on your SG, because Kent Tech M mag, has the tendency of raising, but not by much..but still, just keep your eye on it...however, it might lower your pH, so keep an eye on that as well..
during the process of it, I did not lose any LPS or softies corals, or fish, or any other animals. I have read that corals may bleach, but not to the point of death, and recover nicely; however, my corals never bleached, maybe because I raised it very slowly, thus giving them time to adjust!
up to now, I still dose but not a lot, just enough to keep my mag levels in check, and I have not seen it return!!!
here's a basic link on how to do it:
http://www.saltwateraquariumlighting.com/bryopsis/
GoodLuck and hang in there, you will win, I did!