Suds?

I have some lemon balm, mint, stinging nettle and a couple other plants that seem to have little suds on them. It only seems to be on the lower leaves, but it doesn't seem to be doing anything.

Does anyone know what this might be? Or if it might end up being destructive?

Sorry, wrong section.

Probably Spittlebugs.

http://urbanext.illinois.edu/focus/spittlebug.cfm

Apparently not considered harmful.