You identified the problem, but it isn't someone, it's something--a spitbug to be exact. Just wash off the foam and enjoy your lemon balm. The foam is a secretion that the bug uses to keep it's larvae moist (I think) and isn't harmful to the plant, just unsightly.

Is there any way to get rid of the little spitbugs? And thanks tons for answering! (I love love love my lemonbalm tea!)

I just pluck and squash by hand. They are easy enough to manage that way.

FataMorgana

the only problem with plucking them, is I don't see anything but the 'spit'! Very odd!

If you look inside the spit you will see a little greenish yellow bug. We call them froghoppers over here and they do look a bit like minute frogs. Shoot it off with the hose if you don't want to touch it.

Luckily I haven't really seen them, even looking at the spit....so....I'm going to just hose them off. :)

LOL

Spittlebugs are plant sucking insects that can do damage in large numbers. Look for the adult leafhoppers. If you google 'spittlebug', you'll see many images for you to compare. There are many species.