[yesenadam]

Check out this wonderful, unforgettable spanish movie on that theme, much better than any plot summary could suggest (and also I don't want to give spoilers) : The Uninvited Guest (El habitante incierto)

https://www.imdb.com/title/tt0436374/

Taking the theme to a fantastic extreme is the amazing french miniseries Beyond the Walls (Au-delà des murs), which has a whole bizarre world secretly living in your house.

https://www.imdb.com/title/tt4999820/

[zamadatix]

I can't leave this set of foreign movie recommendations on the topic without a mention of the South Korean movie Parasite which won Best Picture in 2020 https://www.imdb.com/title/tt6751668/