Nice game! I think the idea is great, and the execution worked well. I think you might want to add a mute button to the game, as while the music is nice for the first 3 minutes, after that it becomes quite jarring.
I also think this could work with e.g. hex or square grids quite well, if you want to take the idea further. One problem I kept having is that as the floor becomes more crowded, I have spaces that are just too tight for placing an alien in there.
Overall an enjoyable puzzle game!
My high score was 41 :)