Lois, I tried the elevate Mg levels on my seahorse tank. The horse wasn't bothered in the slightest. Most of my corals took it like a champ, but my Acan frag bit the big one right at the height of the Mg increase. Maybe I increased the level too quickly, and that was the canary coral for the tank....I don't know. All the other inhabitants at the time (mushrooms, ricordeas, leather, zoas, gorgonians, etc....) were fine.
It DID help remove a bryopsis patch I wouldn't have been able to easily get rid of. So far no recurrences. Sort of a shame that elevated Mg doesn't work as well on regular HA.