Haven't read the whole thread sorry, but to me that looks like cal or mg deff.
Don't know Megacrop, but looked it up.
It got 6.5% Ca and that should be all right
Mg is only 1.1% and thats IMO to the low side.
I would give them some epsom salt as soon as possible, start with a folia feed as your not that long into flower.
I mix 1 gram Epsom salt to 1 liter water, that should help on the problem fairly fast.
What pH is your media/nutrient mix and what do you use to pH adjust with?