I would say check with a vet - there are heaps of different things it could be, and a lot of different reasons could be causing it. If the third eyelid (the white part) is showing, a vet should definitely check it out, but it would pay to err on the safe side and get a vet to look at it either way. You wouldn't want Sushi to suffer would you?
I have had a client in the past make an appointment to see the eye specialist who came to the clinic I used to work at every couple of weeks, but in the meantime before she saw Craig, she was treating her dog's eye/s with tea tree oil - because she thought it might help. There's no way I would ever give my pets anything other than what a vet recommends!