Well, there's a thread with a slime here (which  helped with):
http://www.wayofthepixel.net/pixelation/index.php?topic=3249.0

Maybe it wasn't such a good idea to start with a slime.

For slimes, you need to know shading, refection etc. The good side is you don't need any anatomy (except classical slime anatomy).

The lines currently look rather good, but as you are just starting pixel art, I recommend making a smaller version of the monster unless it is a boss (I've been spriting for 2 years and it takes me a long time to do large work...) I did an edit, which fixes many of the issues I had with the monster. I shrunk down the original monster, polished it, added hue shifting, anti aliasing (or selout, I can never remember the difference) redefined the shape slightly, killed the black outlines, added one new colour and got rid of two or three other colours. I also altered the sketchy dithering. I hope the edit helps you:

The large outline is yours, and the small monster is mine.

I guess that means another blow for MS Paint, GraphicsGale, and you >:0

Does that mean I have a strike, or just a severe loss of reputation?

I know, I'm so lazy/tardy/half-asleep/sick in the head.
Anyway, if the transparency wasn't there you wouldn't have moaned... I'll change it in a sec anyways.
Or not, as I only have GraphicsGale FreeEdition (won't do GIFs) and MS paint (messes up the colours)