Eye problem

Hi,
Last week one of my cats developed white stuff in the corner of his eye. His energy was low and he didn't want to eat much. Even light didn't reflect of this eye in a normal way. It was like a light milky film all over his eye. Then, the next day he was better and then the white stuff disappeared. Was this something serious that will happen again? Should I be concerned and making a vet appointment? Or does this happen alot? Anyone else ever see this?
Thanks.

Miro, that white stuff is their 3rd eyelid. It does that when it needs to protect the eye. If it happens again and for more than one day, I would get an appt with the vet.