Simple little game, I liked it! I know you've said that you're planning on improving this game and making an app out of it, so I'll give my honest feedback and critiques:

• Minor Bug: When there were two "grims" on the same side, one of them hit me and it killed me, but the second one also managed to hit me, causing the death/hit sounds to play again.
• Game Feel: Depending on what you're going for in terms of this, you could probably improve the feeling of going back and forth by adding some "gravity-like" acceleration to switching between the walls.
• Sound: I like the goofy-but-spooky music of the game, as it adds a nice frantic feel to it, and I think it could be pushed further by adding some kind of "rapid-stepping"/"running" sound for while the character is running on the wall. Similarly, a dramatic "boom"/"thud" sound could be added to when the character lands on the wall to add impact and sell the vibe of just how bad they wanted to get to the other side.
• Art: Because copyright, replacing the Doom Eternal art for the background would be a good place to start. Beyond that, there's a fair bit of style-mixing going on at the moment. You've got the more bare-bones pixel art style of the grim enemy, the vector art style of the character, and the colourful pixel art style of the moving floor. Picking one style and running with it would make for better visual clarity and cohesion.
• UI: Goes without saying, but stylising the UI to fit your game via typography and matching the colour scheme with the rest of your game would go far.

Keep in mind, with everything said above, I DID enjoy the game! If it seems like I'm tearing you to shreds, I don't mean to, but I made a similar game myself last year for a different game jam, so I'm speaking from a place of experience. Nice work!

This was a surprisingly fun game for such a simple concept, could be really deep with a few more mechanics.