Hey good start! I can see you have grasped a few techniques like anti-aliasing and outlining. I think the biggest improvement that could be made to these is proper shading. They need a light source. At the moment the worm's (?) head looks flat and more like a circle than a sphere, which it is meant to approximate. Try practising shading a sphere first then go back and render his head. Likewise his body is a cylinder.
For the slime, his head is the main form and approximates an egg. Try practising shading an egg first in the same way. Likewise his or her body can be approximated by a cone. Interpolating the pixels between the rendering approximations should result in a complete solid and properly lit form, which is the main thing you are communicating. Everything else just enhances that. It's why traditional art skills help pixel artists so much.
After getting the fundamentals down the next thing is to consider personality and story. How were these creatures born/conceived? Are they male/female/asexual? What planets are they from? What do they prefer eating the most? Do they sleep? Answering some general questions like this can help you to visually shape their character and give them more personality - instead of the hollow slime eyes, we might see a fear-instilling deep red glow or no eyes at all. The expression could also be more menacing if it's going to attack, a wavy mouth usually indicates uncertainty. But expression stuff must come later, once you have got the forms down! Good luck!
