Cuckoo spit - its the nymphs of frogghoppers - if there's not too much of it, ignore it, or you can wipe it off with a cloth (including the nymph which will be present inside it) and if its a serious and heavy infestation, you can get a pesticide treatment, but this isn't usually necessary.

I usually wash them off and squash the little nymphs with fingers. Works for me but as Bamboo says pesticide in heavy infestation.