That was a really neat game! I like the aesthetic and the art a lot. There's also a number of neat level design decisions in here, like how the player can see how slimes work without being put in danger at the beginning of the level, and then they're given the license to figure out that they can double jump in order to progress up the first platform.
There were a couple weird spots of collision (in particular, I got hit by the boss a couple times when it didn't look like I should have.) I also think that the sprite for the slime after it lands on the ground is probably too small; if it's still a damaging hitbox, it should be just as prominent as it was when it was falling.
Anyway, good job on the game!