I personally really like the style, and I think your sprites are very clean. as you said, there is some readibility issues, but I think it comes with the style. one thing I would suggest is trying white outlines--that might clear up some characters from others. (thought I know I'm a bit on the late side for this suggestion, I hope it goes well)

first things first, work on a more neutral background if possible, since due to colors influencing your perception of other colors such a bright green could influence the piece a lot more than you intend

otherwise, here are some thoughts:

i desaturated colors (you were working with 100% or 0% saturation a lot, which made colors very not friendly to the eye) and fixed some anatomy (my version is still very rough tho, i don't have a lot of time)

Thanks, I appreciate your enthusiasm and your gratitude 

since you were talking about removing the edge from your work, I made a quick image to show a good way to do this:



(click on the image to zoom in, and you'll see the technique)

I don't know your familiarity with pixel art, but this technique is called anti-aliasing: it's when you add "midway" pixels between two different colors in an image to smooth the sharp edges. for example, a black outline on a white background would stick out, but with some carefully placed gray pixels, you can trick the eye in to seeing something blurry and smooth (too many gray pixels, and it'll be too blurry).

there are a ton of tutorials out there--I encourage you to look em up

it's hard to tell how your tiles look with all the default tiles "clogging" the screen

but my first notion is that the tiles you made are very monotonous, or in other words they are all very similar. try adding some variation to your tiles, like two or three copies of each tile, to break everything up and have everything look more natural.

for example, look at this stock image of a brick wall:



it has varied shapes and colors; some bricks are cracked and faded; some bricks are more inset than other bricks. variation is what makes it look realistic.

take a look at the default tiles gamemaker, provided, even. there are cracks in the walls, crack tiles on the floor. these are all tiles, too, but they're not the default, generic tiles. instead, these tiles are used to give personality to the setting.

another lesson you can learn from gamemaker's default tiles is their use of hue. the tiles are very saturated (more grey), just like yours. but the default tiles also add a green-ish hue to the wall, which gives it character. grey walls are boring, because they tell us nothing about the building or environment, except maybe that it's boring, too. green walls tell me that the environment is in nature, or that the environment is plagued, or that the environment is on the sea in some way.

the fact is: color speaks volume. imagine if i painted a dog in a bright red and then in a dull brown. a bright red dog would be angry, menacing. a dull brown dog would be subservient and loyal. the same should go for your walls. what are they? where are they? why are they there? this should influence the design of your walls, and also, what little bits like cracks you add to them, too.

both color and detail in mind, i made a quick, rough wall, which should serve to illustrate my points. it's not tile-based, because i'm short on time, but it should show what i meant well enough.



i wish i had a bit more time to make a better edit, but i'm not a great tile-r myself, so id need a while. nevertheless, think about variation and hue to your tiles to give them more interest in the viewer's eye.

its better, but you missed a few really crucial things I did: shading on the head to give it depth (very important, head looks flat), and cleaning up his chest and giving it visual clarity (it doesn't read well right now, hard to tell if it's a glowy thing or a design)

as for your beast :

• line work on the top of the head still had right angles, look wierd / unintentional; i also rounded out the back of the head to give it a more organic feel
• the shading had a lot of banding (pixels "lining up" to create blocks, which makes the sprite look not organic) so I added depth to the shading on the head to fix it and tweaked the pixel placement on the legs to fix those
  
• desaturated the brightest blue (it was at full saturation, which was too bright for the rest of the piece, imo)
• brightened the legs to bring them in line with the rest of the piece--they seemed like a separate element since they were completely darker, and shared very little color with the head. i dont know if that was your intention but in my eyes at least it make it seem strange, especially since the bright highlights made it look slimy like a squid, not a spirit
• fixed the line work on the tentacles following the brightening up, a lot of the lines were sharp (diagonal single pixels or straight lines). try using "transitional lines" insted of sharper ones (so, instead of a border that goes "5-1-1" have one that goes "5-3-2-1", it'll look smoother)
• maded the rightmost tentacle less thick to match the other two
• i wanted to change the middle tentacle, but im not sure what your intention was -- where is it attached? to his body, and pointing up? (that's how i interpreted it, at least, and changed it accordingly) or to his head, and rounded at the bottom? it's hard to read because the shading doesn't tell us

all of this in mind, here's my edit:

yours on the left, wierd tentacle in the middle, my idea of the tentacle on the left

feel free to use in your game to whatever degree you want to. hopefully, my thought process will give you a bit of insight to take your art up a notch. good luck