What could this be?

Hello, it looks like a manganese deficiency. A manganese deficiency detected early can be corrected by flushing the soil with pH 6 water and adding a weak nutrient solution. It's also wise to ensure that there isn't too high a concentration of iron, as iron hinders manganese absorption. If the problem doesn't resolve within a week, you can use manganese chelate. Unfortunately, if the pest appears during flowering, you'll need to do some drastic pruning of the affected parts. Remember not to remove everything at once, as excessive leaf trimming can cause unnecessary stress to the plants.