[fennecfoxen]

That's not really grey goo, in the typical use of the term; that's just a novel agricultural pest.

[freyir]

a hypothetical global catastrophic scenario involving molecular nanotechnology in which out-of-control self-replicating machines consume all biomass on Earth while building more of themselves

It’s right there in the first sentence. Nothing about eating concrete or sand.

[m3kw9]

Can’t create atoms out of thin air, maybe you can but that isn’t enough to replicate rapidly