[nnq]

You don't need to "metabolize useful energy out of concrete, sand, glass, and dirt" to have a nightmare scenario...

Anything that can metabolize lots of kinds of multicellular living matter could can do insane amounts of damage even if it's not a pathogen... for example agriculture is quite frail and we depend on it!

[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