I think you need to clarify the idea a bit more?
are they units? Or some kind of spreading feature or improvement like a forest?
fight thm how? with workers building improvements? Pillaging the nano improvement? Chop them like a forest with a worker? Kill the nano units with some kind of anti-nano unit?
I'm referring to a type of weapon or event later in the game that turns a couple random terrain types across the globe into a nanobot-infested tile, presumably with a name like 'nanosludge' that generates tile improvements over it slowly that spread it further. I came up with some basic rules for something like this:
The first 2-5 infected tiles have nanosludge as a type, and an improvement called something like 'nanofactory'.
If a tile is within three tiles of a nanofactory, and directly next to a nanosludge tile, every turn there will be a 1/4 chance that it will become a nanosludge tile. Water cannot become infected, but coasts become watery nanosludge.
If a tile has the nanosludge type, and is at least 2 tiles away from the nearest nanofactory, then it has a 1/8 chance every turn to spawn a nanofactory on top of it.
Nanosludge tiles damage any units on top of it for 20% of their health every turn.
Nanofactory tile improvements spawn Nanoswarm units (probably using the Planetfall Mindwurm model but retextured) that defend nanofactories and kill off invading units, but cannot leave nanosludge tiles, with a strength of 20. They're spawned with a 1/5 chance every turn from every nanofactory.
Workers can individually clear nanosludge, but it takes 6 turns, so you need multiple workers to get rid of it. Military units can also 'pillage' nanosludge to prevent it from expanding for 2 turns.
The idea is an exponentially growing danger to the world that needs active and immediate effort to stop from expanding out of control and wiping out entire continents, but I'm almost positive this mod doesn't exist.