I knew as soon as I saw the title that I had to play this... I also submitted a game about exploding slimes! (Don't worry; it's very different from yours.) Nice work! Here's my thoughts:
- The level design is excellent. It gradually introduces the mechanics to the player (as it should), the puzzles make some nice use of red herrings, and the later levels have genuinely clever solutions. I'd love to play a version of this with more levels if you two were planning on expanding on this.
- The use of innocent bystanders that need to be left unharmed was a nice touch- though, given the theme of this jam, I was wondering if at some point I'd be forced to kill one of them.
- It's a shame there's no audio, but as someone who also struggles to make time for audio in game jams, I feel your pain.
- Since you can exit the first level without a key, I feel like when the key is introduced, there should be a separate block right in front of the exit with a keyhole that you unlock. Otherwise, it feels kinda inconsistent (at first, I thought the key might unlock a hidden room or something rather than the exit). If you don't want to alter the existing levels, maybe you could have the exit be gray when the level begins, but then turn red when the player grabs the key to signify that it can now be entered.
- The instructions should mention that clicking also chooses which square to explode (for green slimes, anyway). I tried repeatedly tapping space at first, expecting it to explode in all directions like the pink slimes eventually did. Then, of course, i was surprised when the first pink slime *did* explode in all directions, but I guess that's partially my fault for not paying attention to the fact that *all* the red squares highlighted on mouseover rather than just one.

Nice game! Didn't play many levels, though. The restart made me start over all the way from the first level, which made it a chore to get back to the level which I had problems with. Restarted twice. Then I got frustrated. But that aside it was a good game. Very good level design.

It took me a little while to get a hang of the controls (The combination of using the mouse to select units and spaces to blow up, while using the keyboard for actual navigation felt a bit strange at first), but it was a really fun puzzle game!

I agree with @birdwards that the function of the key was a little bit unclear at first - It felt like, I just walked through the same "end of level" warp without a key, why do I need one now?

The puzzles were well designed and smart - I never felt like the answer was too obvious, but it also never felt like I was beating my head against a wall. Good job!

I have one gripe though - The fact that restarting brings you all the way back to the first puzzle felt needlessly brutal, especially when later puzzles are very easy to accidentally make unwinnable. I got through quite a bit before I needed to reset a level, and ended up putting the game down when I realized that I'd need to slog through all the puzzles that I'd already solved, just to get one more attempt at the puzzle I wanted to be solving. :(

Nice ergonomics! You get the point and the principle of the game right away without any explanation! Good move, continue this way :)

Good level design and the "revelation moment" is pretty solid. You have a good grip on puzzle game. Well done.

MUITO BOM O JOGO PARABÉNS QUERO NA STEAM

Took me a little while to finally get there but eventually solved it. The puzzles were definitely doable but like others I was very annoyed that restart forced you to go back to the beginning. On the very last level, I died because after you blow up your first pink slime, it automatically selects another pink slime and when I tried to click the human it ended up blowing up the human. I felt very cheated and had to play through all the way to the end.

That said, was fun. Could have used some music to give it more of an ambience, but it was pretty cleverly done.