[Nasrudith]

To be an utter smartass gray goo long predates us, we call them microbes.

Really Grey goo is a conceptual repeat of bad scientific models like Ice 9. Complete with bad models of chemistry. In this case they forget that the molecular scale is not the same as macroscopic, often assuming molecules are like macroscopic materials that stay still until they act instead of reacting to conditions. Assuming that a self replicating hydrocarbon consumer could break down anything carbon based would be like worrying that the oil spill will start ripping carbon from the air and hydrogen from water to make more oil and create a powder keg world world without any water.

Plus addition forgetting the constraints other than raw resources like dissipation, surface area, and power supply.