If you carefully wipe away the foam, you'll find your answer: a fat little dark-eyed green creature about the size of a sesame seed called a froghopper nymph, so named for the adult insect's squat shape, pop eyes and leaping ability. The nymph stage is better known as a spittlebug.

Froghoppers/spittlebugs insert their "beak" & suck sap through their bodies, extracting nutrients. As the liquid comes out the other end, mixed with soapy abdominal secretions, the insect puffs air into it through a special organ, blowing bubbles. The froth flows down and around the nymph (they feed facing head down), keeping it cool, moist and hidden from you (unless you look). ;o)

Al