I have seen a few of my dogs do this from time to time. It happens once or twice and is then gone for months at a time. It does look like some kind of mucous, but I’m not a vet and can’t really say exactly what it is or what causes it. Benedryl, like other allergy meds, tends to work less the more often you use it. Which is why my vet says to only use it when absolutely necessary so my dog will get the full affect when it’s needed most.