The idea is decent, but the movement is terrible. The character is really annoying to move (low acceleration, slow to stop?). You have to wait 5 seconds to activate things, and you get popped back at the level selection room after each level for some reason. Also you have to be super careful not to accidentally join with a cube you didn't mean to join with. You might want search up how to make responsive platformer movement. (I think GMTK has some videos about that)
(Sorry if this seems mean, I just like to be honest)