It could be fowl pox, but it also could be something such as mycoplasma (MG.) It would be easier to diagnose if there were pox lesions on a comb or wattles to see. Did you get the chickens from someone else or did you hatch them? Could there be a possible carrier of MG in your flock or from where you get them? MG can also be passed on through hatching eggs from carrier hens. If it is fowl pox, it is a virus that has to run it's course over 4-5 weeks. You should clean the eye with saline, and apply either Terramycin or neosporin ointment, or Vetericyn eye gel. The chick looks to have eye bubbles and eye and nasal drainage which can be signs of MG. Antibiotics such as Tylan and oxytetracycline can treat the symptoms of MG. It is a contagious disease that can make carriers of the flock, even when others don't show symptoms. Here is a link to read about MG and fowl pox:
http://www.thepoultrysite.com/disea...ction-mg-chronic-respiratory-disease-chickens
https://www.backyardchickens.com/a/avian-pox-how-to-treat-your-chickens-for-avian-pox