Is this a Magnesium Deficiency?

To me it looks like a trace element deficiency, iron possibly. A small handful of volcanic rock dust applied around the base of the plant every 4-6 weeks or so will provide up to 100, or more, trace elements and minerals that your plant will love and keep it well topped up with minor nutrients. Hope this helps, Organoman.

I would say yes but being so close to harvest the plant itself could just be naturally fading from pulling all the nutrients from the leaves. Even if your feeding the plant may need more, but I'm not saying feed it more. I'm just giving you a bigger picture. If your growing under LED extra Cal-mag is always needed because the lights. Stop during flush.