Well I hope you find the cause whatever it is. I live in the moderate climate of NZ and have to say that my fruit and veg platters go in one morning and are changed the following morning. I have never had any trouble with food spoiling or causing problems. The only exception I would make to that would be anything meat-based (chicken wings, etc) which I would remove after a couple of hours. I have had a couple of cases of apparently drunken sparrows who have been gorging on the fallen fruit from our apple trees when it has started to ferment.
They just need a darkened cage to sleep off the hangover for a few hours (providing I get to them before one of the cats does!
).
Does the 'discharge' look like dried mucus? If so it may be indicative of a mild infection OR it may be that any greens he has eaten have coloured the mucus he produces naturally and made it more noticeable. I would be keeping a watching brief for any other signs of illness. Good luck!
All the best,
Heather
PS If you are worrying then take him back to the vet for another check. Life is too short!