It seems to be Herpes infection there. It can be causes of stress or poor nutrition. Cold environment also causes it. There are few oral antibiotics, which can cure this disease. Take it to vet soon. It is highly recommended to keep it surroundings clean and increase its intake of water and fluid food. You can use eye drops but its better if you ask a vet about those drops before using them. And it is even better to let its vet diagnose the exact disease before giving it any kind of topical or oral medicine.