What’s happening?

P.S. - Try and keep your grow space clean and tidy, all those fallen leaves are a prime place for pathogens to breed!

No, it is burning and not a calcium or magnesium deficiency, which will show up as polka dots on the leaves. Feeding less nitrogen will not show on the older leaves and will take 2-3 weeks to have any noticeable effect on the new growth. There should be more than adequate amounts of cal/mag in the GH nutrients, cal/mag is really only an issue when growing in coco coir. A few funny leaves is quite normal, no plant will ever have 100% perfect leaves, 100% of the time. Your plant is very dark green, going to half strength nutrients for a week or two might be adviseable and will not cause any problems.