Right now you have a few problems, stray or isolated pixels (don't really help much in this case because slime should be smooth.). Your outline, I believe this is an example of sel-out a good one, (or, bad, depending on how you look at it.) at that. Look at this:
As you can see, when creating a sprite that you plan on using for like a game or something, it's usually a bad idea to blend it onto the background. And, also, for a similar reason, it's good to stray from pure-white or black backgrounds, and pick something in-between, so you don't misconstrue how you see your colors. If you're just making this just because, and don't plan on using it for anything, I suppose it's alright then. Most people here frown on sel-out, although it can sometimes be used to an advantage. In this case I'd have to recommend you either remove the sel-out, or remove the outline altogether. (<- don't think you always need an outline, sometimes not having them can yield some pretty effects and make your picture pop, although they are commonly used in games as a means of separating elements, such as sprites from the background, and making sure the viewer's eyes don't get lost or confused, searching for what they need, such as their character, if they are controlling one.)
Here's a quick edit:
I removed the eyebrow feature because in my mind, I just don't see it as viable for such a creature to have, but that is just my opinion, feel free to add it again. It makes the slime look a little menacing, too. I also upped the saturation of the colors by about 40, because I also, simply, see them that way, as opposed to duller, grayer, creatures.
Hope I helped, and good step on removing the dithering, that's anti-smoothness.
EDIT: Damn, Frost, beat me to it, that's a great edit, I could learn a lot from it as well. Mine is very simple.
