The crop is a pouch that holds food. It's on the right side of the breast.
When your chicks have been eating/drinking, you should be able to feel a bulge on the right front. As your chicks eat, the crop fills up. Over the day and night, the crop goes down (as they poop and process food).
Ideally, a crop should be checked in the morning after they have slept all night - the crop should be empty then. Since yours are under a light and eat/drink 24/7 it's harder to tell when the crop is not going down like it should.
If the crop is over full or they have a blockage, then food/water can spill back out.
Your description of them having mucous is concerning - has that stopped?
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