Nice proof of concept! Some sound and music would go a long way.

Good idea, some sound and expantion and the game is done 

am I supposed to break free of the walls and run around in the black and grey area? I had a blobb attached to me which I couldn't shake or destroy.

animation looks nice, fullscreen would be helpful as it is really tiny ;)

I really like the concept, but I don't think I'm able to get hurt, and I can just hold J and kill all the slimes without doing anything else. A Score would also be nice. 
In the edit game menu, you can check the "Fullscreen button" Under Frame options to let it go full screen. And editing the game page after submission shouldn't be against the rules.
Overall, Good job on completing a game. :D