Fun challenge, and the ending is very well drawn and animated!
However, limiting the air jump to only triggering once the slime is already falling down goes against a lot of instincts from prior games, making it a little hard to use, and sometimes the jumps get really wonky when interacting with moving platforms. For instance, noticed that you cannot double jump while in front of one of the blocks that recedes into the wall, making a few of the levels harder than they need to be. Once I wasn't able to jump off of one of the horizontally-moving platforms either.
My misgivings aside, this is a fun and challenging platformer. Good job!