Could it be spittlebug? Look inside the foam for a little green guy.

Sounds like spittle bug, not terribly harmful but definitely gross looking! When my plants start foaming, I use insecticidal soap on them (remembering to rinse well before eating so I don't foam with soap suds).

A strong shot of water from a hose works, too...
hortster

I get this on Basil too. Good spray of water and a nice wash when you're harvesting is all that is needed. I haven't noticed any damage, just an icky feeling when my hands touch it while pinching tips.

I usually handpick the occasionally insect pest like this. A hose would definitely work for these bugs as well.

FataMorgana

grebero, you don't see anything because the larvae (of the spittlebug, a type of leaf hopper) is hidden nicely in that gross gob of spit! I agree that a sharp spray of water (plus fingers) will do the job.