Did you have aliens join the planet that were Festron? Festron lay eggs in the stomach of their hosts and they become festron. 

There are events where they can "guest star".

Plus, in their case, the eggs...

Does it really matter? They replace your citizens, not switch allegiance to another civ. I think they might even keep the citizen traits. I think Stardock has dropped the ball with the Festron. It seems like it really doesn't matter in the long run.

If you really don't want Festron on your planet, then I suggest you build a lot constructors to scoop them up until there are no more. There is a chance that the Festron have already infested every citizen, in which case you will need an extra constructor to scoop up the last Festron once your population grows to provide a new (not Festron) citizen.

Festron = Space Herpies. 

This was what I wanted to say in the original post -

I just noticed that ALL the citizens of my formerly human home world are now Festrons.  The Festron civilization is not even in the game. 

I am not disallowing foreign citizens on my planets.  There are a few Drengins and others on some worlds.  I guess the Festrons are like an "Invasion of the Body Snatchers" race that can turn other citizens into their own race, but, without checking all your planets every turn for a Festron "invasion", how can you know that a planet is infected.  I would think that the governor would notice and be suitably alarmed.  And, even if there was a notification, there should be some choice about how to stop it or let it continue.

I edited the original post with this information and pressed "submit changes"  but it never posted it.

I fixed it. Use clear formatting on the text next time or Ctrl + Shift + V.