3 of my Bantam chickens had what I assumed to be "pasty butt" but when cleaned, turned out to look like this. One of the 3 just passed away about an hour ago. I'm not sure what happened, it was lethargic and wouldn't interact with the other chicks. It was only about a week old, much smaller than yours. I'll have to keep an eye on the others to make sure it doesn't happen again.