Why what is it? Did you over dose it? If you did just do s water change. I have seen mine as high as 1500 and no harm to system or coral. Just take it slow dont do big changes it can do more harm
I would be testing you water change water...…. make sure those parameters are where they should be, especially the Big 3 (Alk, Calc, Mag) Make adjustments in the mixing container if they are off a little. if my mag was high (say 1520 ) and I wanted it at 1400, I would make sure the new water is at least 1450-1460 on the next water change to help bring it down. you don't want to pull it down so much so fast. if you are worried about making the adjustments in the new water, just do the 20% water changes every 2-3 days. it will help, and it will help your other levels as well.
Now if you are dosing your tank to achieve specific parameters, you might want to visit your method to see if there is something that you are doing that might be causing this increase in Magnesium..... cause you don't want that kind of increase on the other 2. that could be potentially deadly to coral. I change water pretty often, but I also have a daily dosing schedule, and if one of the parameters gets out of hand I would be able to make adjustments on the dosing pump to bring it under control. I always thought dosing pumps were for cool factors haha but I love mine!
or
ignore everything that I said and wait a couple days and do another water change