Rust marks on young Lemon Cherry Cookie Auto (2 others next to her are fine)

looks like your having some issues with the medium from PH or lockout. Aldo ditch the Palgron stuff. its garbage. BioBuzz can do it all on its own. Plus mixing some brands may cause issues as well. BioBuzz nutrients do not want a low PH. lowest it likes is 6.2 according to their site and I would keep it around 6.5-6.8 IMO. If the nutrient is fully available the PH is not as important as in a hydro setup. They want to ph low to help off set microbe growth. I've grown out plants fully with no PH regulation in living soil. Not to say their was no issues but got a great harvest on it and did not see any issues in veg, just mid flower. The soil can manage some of its own PH quite well. but needs help long term when it gets bigger. As for the feeding. In soil is best to do a system where its feed, water, water "repeat" or Feed, water "repeat" every other or every 2nd other time. Feeding each and every time can be overloading as you typically don't get runoff in soil and it will build up. Also when a plant wants to shed its own leaves it will do it. This plant is far to small and should not be stripped of any leaves that's only setting you back. It will do this to older leaves with little to no light exposure. I will take off some leaves touching the soil line to prevent diease spreading but I do this once they are bigger. If you plant is growing to compact it could mean the light is to close/strong or the genetics are just very tight on the node spacing. its likely the first one. The discoloration on the upper leaves on the edged leaning in might be from MG issues or PH lockout. Can use some bath salts aka Epsom to give it a boost of Mg if it needs it. Here is a leaf chart to help troubleshoot the issue. https://420life.com/wp-content/uploads/2021/10/Marijuana-Leaf-Deficiency-Chart-scaled.jpg

I would just cut the leaves off and don't mind 😝 They are hindering the air flow anyway 😎 Perhaps the old leaves dont want to exist longer when they dont see light and air 👻

Schwer zu sagen, also es kann schon sein das der Luftbefeuchter ihnen schaden kann wenn dort ständig feucht ist begünstigt es das dort spilze entstehen. Aber es könnte auch Anzeichen von CalMag Mängel sein. Versuche doch de befeuchter etwas weiter weg zu stellen wenn das geht. Und kontrolliere dann weiter. Gebe etwas CalMag dazu das ist immer gut zur vorbeuge, ich gebe bei jedem Gießen 0.5 ml pro Liter dazu einfach um vorzubeugen. Viel erfolg