I would take it to the vet straight away! It could be nothing but my budgie had an orange, sticky discharge from his nostrils that was fatal. The vet couldn't diagnose what it was but gave me antibiotics to give him. Unfortunately it wasn't enough. I don't mean to worry you, that is the worst case scenario but take him to an AVAIN specialist vet and get him checked out properly.
If your vet prescribes your tiel antibiotics, keep a close eye on your cockatiel. If it doesn't improve from 3 days to a week. Take your tiel to the vet again. I believe it was a cold my budgie developed which can be fatal to budgies because they breath through their nostrils, not through their beak. I believe it's similar for a cockatiel too.
It may not be the same illness as my budgie but always get your cockatiel checked out if you have any doubts. Always. It'll get better in no time, I'm sure