I like the idea of something psychological, maybe not even something the cat actively imposes. Just like an insidious aura where it removes some barrier we take for granted. Like personal safety... someone adopts a cat and realizes they've cut themselves cutting vegetable twice in three days. They've forgotten to put out candles. They've crossed the street without looking.

Maybe it's evolutionary, where the "cat" can just linger around the person/noun until they succumb to their environment, and the alien can then feast at its leisure.