I don't see any information on the nutes you're giving this grow ... the only mention is in your comment for the week... Autoflowers do not like a whole lot of nutes so I'm thinking perhaps you've overdosed and created a lockout. At this point, I'd check the pH of your runoff, if that's out of whack you'd need to do a flush. If it's ok, I would definitely add some microbes to help the roots out and prevent any more nastiness from occuring... It's definitely NOT the start of a calmag deficiency - there's not one sign of that so chuck that idea out the window.
There are two micronutrients this MIGHT be the start of a deficiency of - sulpher and/or magnesium - both are solved with epsom salts (add 1-2 tsps per gallon) and in any event will not harm your plants.
All in all, though, your grow looks amazing and I'm not convinced this is something you really need to worry about at this point.
Good luck and enjoy the grow!