Some months ago, I had an issue with low magnesium and very high calcium, much the same as you do. But, in my case, it was due to very low alkalinity at the same time. I'm not certain how things got out of hand the way they did but I think it can be attributed to a bad test kit for one of my water quality tests. Anyway, I did have some problems with coral in the my reef as a result which were addressed quickly without too much damage by raising the alkalinity and the magnesium levels. So, I'd suggest that you quickly have your alkalinity tested at a fish store, or using another test kit for alkalinity, so that you can determine if you need to raise it, rather than assuming that it needs to be lowered. Also, you should, of course, address the shortfall in magnesium.