Welcome to the boards! It's always good to see new people taking an interest in pixel art. I did a small edit on your octo-slimy guy:
These sprites are a nice start (I especially like the squirrel), but they do have some problems. I did a quick check on the slime, for example, and it turns out that you use (in that sprite alone) 33 colours, some of which were so similar that when I tried to extract them, put them into the palette block (the rectangle under the original sprite... dunno what you'd actually call it), I was only able to find, like, 20-ish of them, even at around 8x zoom. The sprite beside your original is a colour reduction, in which I shrunk the palette from the aforementioned 33 to 6, without any loss of detail. In the reduction, I used only colours that came from your original palette.
The sprite beside that is a (very minor) line edit, and a palette overhaul, which incorporates some basic hue shifting. I don't know the exact description, but basically hue shifting involves (wait for it)... shifting the hue of your colours, usually from one end of the spectrum to the other. In this case, the highlights lean towards yellow, while the shadows are more bluish. Since hue shifting generally helps make a piece more visually appealing, it's used pretty comonly (to the point where not doing so seems almost to be the exception, rather than the rule).
For all that, you've done quite a number of things very nicely. There is, for example, a bit of hue shifting going on in the hair of the red-shirt guy, and the fur of the squirrel; you've used almost no full-black outlines; and for the most part, these sprites are easily readable, so nice work on that front. Just get that colour count reduced, and you'll be well on your way
