
It's due to too many tears being produced & not unuusal due the the large eye required by the breed standard
Mine tend to have a bit when they are you & I use homoeopathic eye drops from Doreen Paige & her stain remover to keep the eyes clear & remove the staining
Never heard of the food being blamed
If one of my dogs has a problem & use the drops on a daily basis until it clears up.
This is different to conjunctivitis is a different matter & needs veterinary attention of course