I'd leave it alone, 1500 won't hurt anything.
Stop dosing any Mg additives until you get it where you want it.
Also, double check the test kit and make sure there is no error there...
It's always prudent to double check your test results before making any adjustments. If you're using a "Reef/Pro" type salt mix, i.e. Reef Crystals, SeaChem Reef, you'll find that their mix is near 1500ppm when mixed to a SG of 1.026/35ppt. Another possibility is non/mis calibrated salinity measuring device causing you to add too much(or too little) salt mix to your make up water, and by extension too much(or too little) Ca, Mg and Alk.