Really cute graphics, it fits the simple 2D style really well!
The rust slowing down the character is a neat idea, however as soon as it kicks in, because it's a platformer based on (sometimes a bit too unforgiving) jumps, it starts a negative feedback loop which makes it impossible to progress further after dying just once.
I would suggest having a different mechanic for replenishing it rather than that being equivalent to HP - i.e. when out of combat, press and hold a button to restore yourself!