Those on the sweet potato are technically called trichomes and they are exactly what the links says: an outgrowth on the leaves of Ipomea sp. plants that is apparently tied to stress conditions. Again technically it's an abiotic condition, meaning not caused by a pathogen nor by a pest.
So far so good.
I think the problem with the cabbage is another matter completely: it looks like aphids fed on the leaves but aren't there anymore. The substance looks a whole lot like honeydew (the byproduct of aphids feeding) but without looking at it under the microscope or at least holding it in my hand there's no way to know.
I have had it happen only once and on a single plant (a cauliflower if I remember correctly) but I hadn't given much thought about it until I saw your pictures since the plant didn't seem negatively affected. All I can say is since I grow Brassicae in cold weather only it surely wasn't heat stress.
I hope to dig out more on this strange condition or that somebody else will be around with a neat explanation.
It will save me the trouble.
