Grey Goo is like a Horde of Alien Locusts, only replace Alien Locusts with Nanotechnology — and instead of grass, they "eat" anything. Or, if the protagonists are lucky, just anything mineral or electronic.
They're worse than alien locusts. Grey Goo destroys resources by turning them into more grey goo — more nanomachines. It's The Virus for nonliving things — though it may be able to take down [...] [tvtropes.org]