I'm showing my general frog ignorance here, but what makes the skin of a green frog slimy?

I had always thought it was a secretion from the skin, but was recently shown a picture of a green frog (sorry, it's not on the web) and the photographer believed the frog was spreading goop from it's mouth to all over it's body. The picture reminded me of a Grey Treefrog shedding and eating it's skin, but he thought the goop was coming from the mouth, not going in.

So a second related question, do Green Frogs shed like Greys?

Thanks in advance!