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I'm going to go with some sort of caterpillar or worm like larvae, likely green considering what it is eating. It also looks like it hangs our on the bottom side of the leaf since I don't see any poop from it on the leaf. As for how to fix it....Neem oil maybe, spinosad maybe, or Pyrethrin maybe. Maybe one will work, maybe all will work, maybe non will work, or maybe it will work for a while and then they will develop a resistance to it. This is just the nature pests and pesticides. Another route to go that I like is to grow companion plants near by, like carrots, radishes, pumpkins, and sunflowers. The more species the better. the flowers attract beneficial insect and the leaves of the radishes attract the pests more than the cannabis and in so doing effectively act as a trap, but unlike most traps the the "trapped" creature is probably happy munching on your radishes. Don't use any type of insecticide on anything you are going to ingest obviously, so if your plants are starting to flower don't spray them. Also, how bad is it? Is ever leaf of the plant getting demolished or is it just a few leaves on the bottom?
The bug can't be identified without a picture at least, more info would help - such as how many leaves/plants affected, is grow indoor or out, in veg or bloom, etc - for context.
That said I would hazard a guess it is green and lives on your plant so is probably camouflaged. I have seen similar damage in my garden and only found the caterpillar (see @CoastalKind above) because it moved, it was that invisible to me.
If it's not living on your plant you either have to stalk it, at night (as you say), or make the leaves unattractive to it.
If you're not in flower, spray some Avid. I've never seen a leaf munching/sucking bug that could survive it. It's systemic, so it goes on protecting for weeks after application.