Sorry, not buying the "hidden ice on the Moon" premise. Water ice in vacuum (of which the Moon's surface atmosphere is near enough to count) sublimates quite rapidly, and would not persist for any useful length of time.

I think it's very likely that humans, given advanced alien technology and the ability to use it, would become the implacable scourge. Wouldn't it be interesting, though, if the Earth-humans and the dying race and the scourge were all humans at different stages of development? You'd have to have some sort of time-travel to make it work, but given a race that can fake a couple million years of evolution all over a planet, that's not so far-fetched.