Need some help !

I have to add, would not recommend using hydrogen peroxide on flowers though havnt done that myself atleast, so dont spray it while in flower but could clean each leaf if mold is issue, sorry for the long speach here, in the soil its good to use as a flush tool and cleaner for roots as mention so try and see if helps

I would try use hydrogen peroxide both in soil and on plant itself, it Will kill any mold and also kill any insects etc, but keep in mind it Will also kill off a lot of the micro/bakteria and so on in soil, so you Will have to add this after treatment, hydrogen peroxide is something all growers should keep in storage, its Even usefull for organic growers as it become air and water as soon as its done doing its thing in soil, witch is good also cleaning roots for New nutrients, good luck

Looks like Nitrogen deficiency to me late stage on that leaf. Also attacking lowers on your other plants on your grow dairy I would hit them with feed.

Nah—- was this a lower leaf? Sometimes leaves just turn yellow on the underside—- if it’s just one, I wouldn’t worry.

Neem oil is for pests (insects), this doesn't appear to be insect related.