its called cuckoo spit. inside is a little green insect called a frog hopper. it is a sap sucker. I wash mine off with the hose pipe.

Doesn't usually cause much harm - Seaburngirl's hosepipe method works if you don't like the look of them or if there are loads.

I would only bother washing it off if you were going to use a spray for cooking. It is quite harmless, and if you look around you'll probably see it on lots of other plants too.