Do you have other chickens? Are any of them coughing, sneezing, or making weird sounds? You may have an injured or infected eye, a respiratory disease which can be bacterial, viral, or other. Before you start medicating you might keep trying to find out the cause. There are different drugs for different illnesses, even the different respiratory diseases. Salmonella can cause pus drainage from an eye, but also other things. Isolate this bird from your otheres until you can figure it out. Try this website to look at different symptoms: amerpoultryassn.com/respiratory_disease.htm and even google common poultry diseases.