It might not be only one thing, but based upon the original description, I'd say it is at least partially "honeydew", a sweet, sticky substance exuded by plants that have been particularly well grown.
I surmise (AKA "guess") that when a plant has plenty of food and light, and no undue stresses, it builds up its energy stores (sugars) to a certain level, and then must "dump" some, as there just isn't room for more, and that's what we see a leaf edges, undersides, and even on spikes.
I have also heard that the sugars might be pollinator attractants.
A caveat is that the sugary stuff can also attract pest insects, so some wash it off with slightly warm water on a paper towel. (I don't bother)
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