Honestly, we really don't know a lot about TFAW.
It may have limitations to what it can/cannot infect/copy. I imagine it could infect a predator easily enough, not sure about the Alien. no idea what it has for an immune system, or exactly what they are made of. Event he Predator is still not proven it could infect.Then again, maybe it could. We don't know for sure, as we have had only one specimen to observe, and it infects human and mammal life easily enough.
It may also have an Achilles' heel.
If it has none other than fire, and can infect anything, then that makes it virtually unstoppable. You could nuke it, but if even a tiny bit remains...