Good job with your first piece. Quite an ambitious one too! In terms of pure pixel art technique, I think there's a lot of noise here in the form of individual pixels that don't contribute, but detract from the overall quality. When working on a small canvas, it becomes even more important to use bigger clusters to make every shape recognisable. On a more general level, it's kind of hard to see what the red is supposed to be. Is it lava? Then why doesn't it cast light on the surrounding rocks? And on another note, while the cave (or is it an asteroid worm? ) is brighter near the opening, the red band maintains the exact same hue.

Looking forward to your next piece. Be careful with those pallets.