Nice, fun concept, but a few notes:
1) would be nice to see currency at all times, not just when looking to buy upgrades
2) some kind of reward for finishing a level would be cool
3) I don't understand why there needs to be an efficiency upgrade for not moving, it is optimal to be always moving afaik
4) it wasn't immediately obvious that the colored currency comes from the puddles, that took some trial and error to find out