looks like pasty butt; poop dried up on the feathers around the vent. The problem in chicks is that it can prevent more poop from coming out and that can kill them. I use a wet wash cloth and drip a little warm water on it to loosen it up and then wipe/pull it off. clip or pull the fuzz off around the vent to help keep it from happening again. I feed fermented feed and except for unusual cases don't have it. You can also add unpasturized apple cider vinegar, with the 'mother' to their drinking water; I think it's one or two tablespoons to a quart. You could also give a little plain organic yogurt with live cultures or some cultured buttermilk; these things all help add good bacteria to the gut and help prevent pasty butt.
In my experience chicks that have had it have had some kind of stress; they got chilled (maybe the kids kept them out of the brooder playing too long) they had a rough hatch and had to be helped out of their shell, things like that. Example I had a chick that was half unzipped for more than 12 hours when I realized it had shrink wrapped. I had to wet the membrane and tear it some. Some of the membrane and shell were stuck to the chick really bad. The next day it had pasty butt. I got the glob of dried poop off of it and about two minutes after I put it back in the brooder it had a humongous poop, even a speck of blood on it and it cried. But then it was running around eating and drinking and has been fine since.