Strange coloring on this one leaf?

Hi buddy as long as you are not in bloom, spray the underside of the leaf with neem oil

If you run boiling water through your medium before planting kills all bugs in the soil or peatmoss

Hi mate. Yes, probably fungus gnats are the cause of such leaf color. Currently, I have the same problem, and one of my plants is showing the same color as yours. Also, the plants with more fungus gnats are the ones with the most slow growth. Try to water with neem oil to kill the larvae that can affect the root system. Take care.

Looks to me like it was a little to close to your sun when that leaf was growing out. Sticky strip for gnats. You can also take your vaccuum in there get the little buggers flying and suck the little buggers up to help get rid of the mass of them

I looked at your grow. Have you been feeding her with anything? How's the pH of your water? Are you letting it sit out 24 hrs to let the chlorine go out? Looks to me like you need to start a feeding cycle. Start at 1/4 str and if she's happy with it slowly ramp it up. Cheers and happy growing!