The game seems pretty polished, although i went ~2 minutes without seeing a single alien. The concept is smart and was executed relatively well. Good job! 

Cute art.  I like the idea of an imposter cat amongst the rest. I noticed the Alien always arrives from the top of the screen, so it becomes quite easy to spot em early on. Perhaps if they moved around a little more it might be more difficult.  Currently you only look around on the ground for the cats so a lot of the screen space is not used.  Perhaps have cats walk back and forth on tables and counters  to make the player have to look around someone else besides the bottom of the screen.