My male Golden Retriever is 1-1/2 yrs old. has no health problems that I know of, but has very slimy saliva. After playing ball for a few minutes, the ball is slimy and kinda foamy...Does anyone know what could cause that?

Dogs that play hard or are exerting a lot of energy may foam at the mouth. 

When the dog is exerting a lot of energy,
he may salivate heavily. As he exerts energy or
effort, his breathing becomes rapid and the air of the heavy breathing turns the
saliva into a foam.