Edit:
I changed a loot of stuff, but thats mostly so you can pick up on a lot of important stuff. So firstly you had a lot of banding, beginner's mistake. If you don't know what it is id prefer you look up what it is instead of me explaining it here.
Secondly, shading. I see that you made a shadow below the slime, that means light is coming from above and you should have a shadow on the slime as well.
Black lines, I saw that you had both black outlines as black "inlines". so the eyes and the lines around the mouth are black as well. I would say that 99/100 times black inlines are very ugly. Either give them a lighter colour like I did with the eyes or just remove them like I did with the mouth.