Music is too loud, quit button on title does nothing, controls are never explained, you should add controller support for splitting (jump and movement work with controllers). There are a lot of collision bugs that are caused by fusing. I would recommend using a collier that is the size of the fused slime as a check if the post-fusion has space to spawn.
The art was very solid and the game looks fantastic for a game jam game, I'm thoroughly impressed. Good job :)