Solarpunk

This sounds like the beginning of an apocalypse movie.

Can they promise it will only eat that? Also what's the byproduct.

https://en.wikipedia.org/wiki/Gray_goo

Some version of this concept has floated (pun intended) around for a while. Thing is that plastics are an energy dense "food source" and once something evolves a way to metabolize it they'll have a huge advantage.

It's like how glyphosate tolerance has appeared in weed plants already. Evolution is always looking for an angle.

Ill Wind, by Kevin J. Anderson & Doug Beason

Not necessary, normal bacteria&friends are already doing it. Also funnily enough there is an increase in biodiversity because of the Great Pacific Garbage patch because sea life can attach to the trash, creating a new ecosystem.

Nature do be amazing like that.