Ultimate Fatal Octopus is a Touhou Project Fangame developed and published by SlimeSmile, one of the developers behind Abyss Soul Lotus.
It is a single stage vertical Shoot 'em up and a collaboration with a Vtuber called Syrena (likely from Bilibili if my short research is correct).
Short, fun, and easily replayable with it's high score system and online leaderboards.

Game store page - https://store.steampowered.com/app/26...

Edit: This video footage is of the launch version. As of May 3rd 2024, it has since been updated. Update history can be viewed on the game's Steam page.