I am not certain what you are seeing, however there are two distinct possibilities: there is corneal opacity or edema (the clear covering of the external eye) or the third eyelid, also known as the nictitans, may be extruded.
Corneal damage must be treated now, so a vet visit is in order.
The extruded 3rd eyelid is not necessarily an emergency, since many conditions can cause this structure to cover the eye. And it is possible that there may not be an eye problem that is causing this condition. Fever, upper respiratory infections, medication, environmental irritation, ear problems, etc. can all cause this 3rd eyelid presentation. If not better by the time you read this, then visit your vet.
Dr. Van Lienden
Dr. Raymond Van Lienden DVM
The Animal Clinic of Clifton
12702 Chapel Road, Clifton
Virginia, U.S.A. 20124