Aside from the boring start, I really really enjoyed this submission and hope you consider my feedback as somebody who would definitely buy this as a fleshed out release.
- Most solutions resort to stacking health for passing through spikes
I realize a huge gimmick is that health is tied to the growing squares but I hope you find a more interesting way to incentivize keeping those blocks big in order to reach the exit. Maybe the growing blocks could be shifted back into the ground, and then they function as a bridge to cross over gaps? Maybe introducing enemies could allow you to use growing squares to block the enemies from reaching you?
- There should be some form of animation to convey growth/inability to grow
I do not think under regular circumstances that it is intuitive that a block can not grow if it is blocked by a wall. This tripped me up while playing and honestly wasn't satisfying without a proper animation like having the blocks attempt to stretch but they can't.
- The player sprite looks like a growing cube
It was personally confusing that I'm playing as what looks like a growing square but I don't actually grow myself. This was reinforced by seeing the HP suggest that with more health I will had more blocks around me. I don't know how you'd do this without completely rebalancing the game, but that idea of growing yourself like the growing squares do sound like a really fun gimmick maybe for a level or two.
- Needs more level gimmicks to change up the gameplay
I'm sure you only included as much as you did for a jam game because of the time limit, but it goes without saying that more gimmicks would be really fun. I could see a portal being especially cool as it allows for different angles for the electric beam and could allow for looping through a section of spikes that's normally only able to survive going through one end. I also would've loved to see a corner mirror object that can reflect the electric beam at a 90 degree angle allowing for control over growing squares that are currently blocked off or something like that.
Also consider adding new growth gimmicks like perhaps a growing square that only expands up and down, or a growing square that adds spikes around it instead of walls. I think this is such a solid foundation that it should be easy to add fun new gimmicks that allow for creative puzzles and solutions. More ways to heal and harm is a simple way to add depth on top of this formula. Adding a shield could be cool too as a way to take damage without changing the room.
- The yellow/white color indication isn't very clear
While I am not personally colorblind, I was struggling to tell what was yellow and what was white at times, as well as remembering which color meant it was tied to my health. Perhaps a greater difference in color along with some animation/graphic to indicate what is now affected by your HP would do wonders for accessibility and game feel.
- A turn counter/timer for beating levels would add incentive to puzzle well
Death is a fine punishment for failing to solve the puzzle, but I was hoping to see some way to show off that I beat a puzzle in an unexpected way or with a clever route to reduce my turns. I definitely beat a few puzzles here with more health than I thought was expected because I did a weird route so there seems to be some room for different solutions (which I think is great).
I wrote all of this out because I loved the concept that much and truly hope it gets explored more in the future. I understand you/your team might not want to put in all this effort or even bother with a more complete release outside of jam fixes, but man would I just eat that up. I'll even offer you my help as a pixel artist if you need the assistance on that end, whatever it takes to see this polished into the absolute gem I know it could be!