I grow in soil and water with bubbled tap water and I stop using cal/mag like 3 weeks before harvest.
I also stop using molasses 7-10 days before harvest, ive used it up to harvest before and the buds had a molasses smell to the taste, It was gross.
I have a friend of a friend who was a friend to Jerry Garcia, haha, and he stopped using all nutrients and cal/mag additives 2 weeks before harvest and supplemented with a half of a fresh juiced lemon and a tbsp or two of Sucanat (ph balanced to 6.7 in water) and his kush was quite delectable
But enough of my rambling, no theres not enough mag in molasses to benefit the plant, but if your talking about using molasses during your water only days before harvest, then yest stop using cal/mag for the 7-10 water only days before harvest.