Disease?

If the leaf was overlapping or "underlapping" another leaf, this is what it can look like...........not an issue really. One leaf out of possibly hundreds is not a real problem and unlikely to be a disease of any sort, a disease would affect multiple leaves at once. Remember too, no plant will ever have 100% perfect leaves, 100% of the time.

hard to tell from the pic alone, but I would guess that it is like AsNoriu said. a bit to much N. and maybe some other things adding to it. I would not pluck the leaves off they are still producing energy for the plant, and it will be harder but not impossible to tell if it is progressing. also if the leave was lower or higher on the plant can give you some clues as to what it is. it might also help to ahve pics of the whole plant not just a zoomed in leaf. https://420life.com/wp-content/uploads/2021/10/Marijuana-Leaf-Deficiency-Chart-scaled.jpg This is a nice leaf chart, use it as a tool to help identify issues.

Poo stains for sure.

Overfeeding is first idea. Heavily too dry RH and intense light in environment - second. Would bet my money on first. Personally would reduce Nitrogen or cut it off for a week of so in such case ....