Do you or others who are caring for the chicks have long hair? Have you or others been holding a chick when it pecked at your hair? Because that's what this appears to be, a hair that a chick swallowed and it went through its system because hair cannot be digested.
To verify what this is, wash it first to remove the gooey stuff surrounding it. If it dissolves completely, it's just poop as @Eggcessive suggests. If the thing is solid and has segments, it's a worm, which is highly unlikely in a week-old chick. If it's a hair, washing will reveal it.
If it turns out to be a hair, this is potentially deadly for chicks. A hair can cause a blockage in the intestines or even get wrapped around the tongue. Be careful no one is leaning over the brooder and a chick snatches a hair.