Sea hares do not touch bryopsis as it has a toxin. I wrote an article long time ago on what you can do to help eradicate your problem.
http://blog.fragd.it/2008/05/09/step...beat-bryopsis/
But some people have had success with Kent Tech-M, because of its impurities.
http://www.kentmarine.com/products/k...-magnesium.htm