I think my main gripe with this game is the lack of feedback. A simple sound/effect when possessing and cancelling the possession would go a long way.
Another nice thing would be a better visual of what happens when you press a button/shoot an eye. There was a level where I had no idea what either of them did (and then I was killed by spikes, but that was mostly my own fault).
For the first possession level, consider moving the skeleton to the left side and have the button on the right. I spent an embarrassingly long time wondering why I couldn't possess the slime (button), before I found the skeleton.