We have been getting a bunch of small white cotton-looking things on the top surface of leaves on our Hydrangea plants (it's summer here). They do not move or fly and appear to be dead but they seem to have legs of some kind. It seems the most likely thing they are is aphid casts / skins. My question is - how do they come to be on our plants - are they eggs that have hatched. If so, what do they look like before they hatched i.e. there's nothing obvious on the leaves except the white stuff and it just appeared out of nowhere.