That looks like it's vent is covered. If it is you need to get it cleared ASAP with a warm wet washcloth. Just wipe gently until it's free and wear a shirt that can handle poop. If they had some that couldn't get out, it tend to just split out once the pressure is gone
Hmm, do you have any way to separate them in the brooder? Or you could try using something to hide it? Honestly I've never had a problem with my chicks with umbilical cords