Oh! those guys are probably meadow spittlebugs, which are related to cicadas on the other side of the rockies. the adults are black flying insects that mature every summer and lay eggs all over the place, and then next spring those eggs hatch. cute little green nymphs hang out in the dampest parts of whatever plant they were laid on, and they suck sap out of the plant in the same way as aphids do (but without being nearly as detrimental to the plants as aphids are).

once they begin feeding, it creates that white foam that does lots of great thing for the larva - it helps to keep the larva from drying out, it helps to hide it from predators, and its bitter so if a predator does find it then hopefully the predator will lose its taste for the larva once it takes it's first bite.

stick your finger in that foamy mass and you'll find possibly the cutest little larva you've ever seen - the 'eyes' of the thing are really just fake-out spots on its abdomen to encourage any predator to take a bite of its butt instead of its head. right now i've been finding spittlebugs in their 3rd and 4th instar, a fancy name for larval stage, and they've got 5 total before they pupate and become adults.

Yup. those are spittle bugs. swipe some up with your finger and muc it around a bit and you'll find a wee green bug.