Hi there! I found your game because it has a set on RetroAchievements! I enjoyed it and I'd love to see more of it as I think this could go much further with the alien unlocking more abilities to destroy or navigate through obstacles.
A form of melee attack, a form of ranged attack, being able to crouch/squeeze through gaps, being able to grab things above you and climb on ceilings, being able to use a tentacle to activate certain switches or something: there's plenty that could be added without straying too far from your original concept... if you ever wanted to update this to make it a longer game or if you made a sequel/remake.
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Things I thought were well done:
* I think the graphics and idea are great.
* The sound design was good.
* The text intro and outro were a nice touch to make it feel a bit more professional
* The idea of having a monster ending or pacifist ending
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If I had any notes to help improve this or your next game:
* I did think the climbing controls were a bit iffy at times although it wasn't too frustrating.
* Some of the smaller platforms and narrow jumps were slightly difficult to judge because it felt like the creature's hitbox didn't necessarily seem as obvious as it seems from the visual. This only really applied for one-tile platforms or one-tile spaces to jump through.
* I think a slight indication of where a door opened would add that extra polish. If you don't want the screen to move over to show the player the door, even just an arrow on the screen for a moment or a blue glow on that edge of the screen for a moment or something might make a big difference for that.
* I loved the intro giving a bit of story to the game, but sadly it just made me feel like I wanted to know more. I think if you do expand this, having regular little updates from the humans would be awesome and could even be affected by whether the player chose to be a pacifist or not. Like after getting arms, maybe we cut to a scene where one human says "Hey, George. Do you know what happened to experiment 2136? The glass container is empty." Later on, when you get to the next cutscene, it could either be "George is missing. He didn't get me that report I asked for." or "George, this report shows that we might have an escaped specimen!" depending on whether you killed that human or not.
* Finally, a tiny bit more in the outro also would feel much more complete. The intro gives a sense of scale and purpose, but the outro just feels like a video game ending. I think something like "The creature was a monster! It killed all the staff in the facility, causing a planet-wide lockdown. There is talk of the planet being destroyed before any further human death comes from this" would make it feel like the player's actions had actual consequences. Alternatively, on the other side of it, if you didn't kill any humans, you'd get "The creature just wanted to escape. It did not intend to harm its captors even though the humans were cruel and narrow-minded. There is talk of the facility being shut down because of the mistreatment of the specimens. The planet may be left to continue life in peace after all."