Avocado seedling leaves seem to be being eaten at.
It looks like something is chewing away at the leaves, but when I look closely at the leaves there's nothing there.
Certified GKH Gardening Expert
Some pests are very tiny and difficult to see with the naked eye. Also some pests like slugs or snails only feed in the night.
Neem Oil is a good treatment and the article below has more info for slugs.