This looks like a specific kind of... fungus?...aging?. I can't find an English word for it, but it looks like the black spots you get in clothes when you leave them in a wet pile for too long. In Dutch, is called "spocht".
a lot of the reviews for the kettle I bought on amazon had reviews that complained about this stuff, but we buy filtered water in those refillable 5 gallon jugs and havent had an issue, we have hard water out the tap so i try to avoid using that for cooking, but also I wouldnt trust a used kettle, Id imagine its like the hotel coffee makers where people probably poop in it