You need to see your vet. It indicates and illness, infection or disease that needs treatment. It indicates things like conjuntrivitis, URI, herpes, and even FIP. See your vet.
If you don't have the money to go to the vet there is a old way to do it . Take a bag of Camile tee, wet it down and gently rub out the eye. Repeat 3 times or more every day. The camile tee is good for healing all so good for stomach ache.
Theres not really a cure just keep getting it out I no its gross but then it starts to go