Odd that only one mag. product was successful: could it be that they use a slightly different chemical makeup, and it's that specific formulation, and not mag. directly that does the deed?
I've been slowly growing an infestation myself, though siphoning, as well as tons of coraline to deny it surfaces, has helped. And now some of it seems to be whiting out and dying on its own, as well as these strange white "trees" on my live rock: anytime some bryopsis starts growing in their area and starts to overgrow them, I come back days later and the bryopsis is literally gone without a trace in that area. Bizarre.