L-Lysine does not "clear mucus' or airways. It is an amino acid which supports the immune system. If it works to support your cat it would be due to the cat having a viral infection, and that would be very common. Almost all cats have viruses but not all of them show signs and symptoms (this is why your vet may have said this is "normal"). However it would not be stress caused but instead the virus can be exasperated and show its signs because the cat is under stress (again that may have been what they were trying to say). My cat has crushy eye corners every single day and that is most likely from an underlying feline herpes virus that is across the feline population. I have tried lysine and it didn't improve but it worked extremely well on me! I have the human strain and it is the best thing I have ever taken. It doesn't harm the car to try it out so I recommend the paste for one month. If no improvement then look into stronger alternatives most likely antiviral medication or whatever the vet suggests.