What's causing This??

As usual, I'd recommend using some calcium, as it kinda looks like you're deficient. Buffer, neutralizing pH. It's very difficult to fuck up with calcium.

Just a little info. You can pour 20 gallons of water through your girl and that is not overwatering. Overwatering is doing that all the time and never letting her dry out. Cheers

The yellow leaf is nothing to worry about its just old and she has consumed it for nutrients just pull it off. it will only drop off soon anyway. The green leaf in your picture i would guess either 2 things. 1. Not enough oxygen getting into the root zone. 2. could be down to drastic ph fluctuations would be my 2nd guess as a less sinister guess. Im sure she will be fine the majoirty of growth on her is healthy so she has more than enough to continue to grow. She should be fine keep up the great work :) Happy growing 👊

🥦🥦Note: I top treated the soil, with magnesium salt, bone meal, and Kool bloom, let's see what happens.. happyy growing 🥦🥦🥦