Disclosure: I am not a vet.
Gliders get sleepies/eye boogers like people, it is there to help keep eyes closed while we sleep. It is probably nothing. If he is opening in by himself quickly and it doesn't seem to upset or bother him, don't worry about it. If he begins to take longer to open his eye or he becomes panicked that he can't see out of one eye, take him in, but don't worry about something that small by itself.