I would try to get testing to find out if it is MG or coryza. An infection may occur if they get something, such as a foxtail or seed pod stuck in the eye. All of those may cause a cheesy exudate or pus. Does the eye or beak smell bad? Have you seen watery or bubbly eyes, congestion, sneezing or coughing? Is this chicken new or has it been exposed to a new flock member? Are you treating with an antibiotic?