Your dog has a yellowish green discharge from his penis what is it?

It is often referred to as "smegma." You can view wikipedia's information on smegma by clicking on this link: http://en.wikipedia.org/wiki/Smegma

The discharge from the prepuce is probably normal. There is a thick exudate, referred to as smegma, which is thick yellow to yellow-green in color and is normal. For some reason, it may seem to suddenly accumulate in larger quantities in some dogs.