dunno, corals are weird. For me, unless it's a giant coral, I tend to leave them alone and let them die. Sometimes, they'll miraculously come back, if not, it's liverock to mount frags to and grow out to swap with someone down the road. Fungia's propagate by dieing as a polyp, then "out of the ashes" rise maybe a dozen mini fungias that then spread around and grow.
Sorry for your loss though. It probably wasn't the magnesium, so I wouldn't worry about that test kit. I have the test kit, but I test maybe twice a year (though I should test and dose mag more frequently since I use quite a bit of 2 part).
Do you know if your tank's temp has been spiking at all with the weather? Still everything else in the tank seems normal/happy? Who knows. Sorry man. Sometimes this hobby just
