Could use some improbement
-I really liked the first three levels and it is those levels that really let the gameplay shine. I liked the idea of trying to strategically grow yourself outwards instead of just upwards, but once the pixels started falling to fast the strategy goes out the window and it devolves into frantically waving the mouse back and forth in a mad attempt to stay alive. It is a great idea, and the firth three levels really show me how great of an idea it is so it is too bad you couldn't keep that going for longer.
-Perhaps instead of raising the difficulty by increasing the speed, perhaps add some pixels that are detrimental to your progress, perhaps a black pixel that destroys other pixels. That might add some neat strategy as well. Just an idea though. I hope you take the time to improve this idea!