Something is going on? Magnesium deficiency?

So I'm having an issue here, I think. From what I've read, and seen, I think this looks like a magnesium deficiency. But I've also read that that typically starts at the bottom leaves and goes up. Well, the majority of this are on the upper leaves. Not sure what to do . I don't want to give it the wrong stuff and make things worse. The plant still seems to be healthy otherwise.

And today is day 43 of flower.

your plants are beautiful. they are maturing correctly. you should be tapering your nutrient down like the directions suggest?
doing so would result in some deficiencies, naturally occurring as the plant uses its stores to finish its cycle, leaving behind a properly grown plant needing little to no curing after dry to taste like a million bucks and burn even better. grow organic, dont panic