Mareks is almost everywhere according to information I have read. Many chickens seem to have natural immunity to it, but I still like to get vaccinated chicks. Half of my flock is vaccinated while the others were not, and I have never seen the disease. Occular Mareks is one of the four types of the disease, and your flock has been exposed if she has been with them. Have you ever had one die of a tumor or paralysis, or for no apparent sudden symptoms? There is not anything that can be done about the disease, but I would just continue to watch her and make sure that she gets enough food and water. Since I've never treated Mareks, I'm not sure if both eyes become affected, or if one does, if that is truly Mareks. Here are some good links to read about the disease, and hopefully it hasn't affected your chickens:
https://www.backyardchickens.com/a/the-great-big-giant-mareks-disease-faq
https://extension.unh.edu/resources/files/Resource000791_Rep813.pdf
http://www.merckvetmanual.com/mvm/poultry/neoplasms/mareks_disease_in_poultry.html
http://partnersah.vet.cornell.edu/avian-atlas/#/disease/Marek's_Disease
https://www.backyardchickens.com/t/894589/moving-forward-breeding-for-resistance-to-mareks-disease
https://www.backyardchickens.com/a/the-great-big-giant-mareks-disease-faq
https://extension.unh.edu/resources/files/Resource000791_Rep813.pdf
http://www.merckvetmanual.com/mvm/poultry/neoplasms/mareks_disease_in_poultry.html
http://partnersah.vet.cornell.edu/avian-atlas/#/disease/Marek's_Disease
https://www.backyardchickens.com/t/894589/moving-forward-breeding-for-resistance-to-mareks-disease
Will keep monitoring her and the others. So thankful for this web site. Dingbat lives on! 
