I don't know if I'm qualified to give feedback, but here's some things.
Number one, the pacing. It's going too fast for me to keep up. He's in pain, then he's in all black with no transition in between. I would prefer you slow down, and the run-on sentences don't help. For example "After all that pain subsided, I was barely able to open my eyes and take in my surroundings and the first thing I noticed was that I was black, my arms were black, even my legs were black, every part of my body was completely black." is at least four sentences in one. You could shorten it and cut it up to make it easier to read. For example, instead of that, I could do, "The pain finally subsided. Breathing heavily, I slowly opened my eyes. Taking in all the surroundings, the very first thing I noticed was that something was wrong. My arms were black. My legs, my chest, my entire body was completely black,". That's five sentences from your one. And I could cut it up further with even more visceral detail to make the reader feel the slow, creeping sense of jarring discovery. Of course that doesn't mean that run-on sentences are always bad or good. There is always a place and time to use particular structure crafts. That, along with creating all the right emotions, is the most vital and hardest part of writing, or so I've found.
And I feel like that it's supposed to be first person but feels like second because of the past tense. It's a little jarring. For example, instead of something like "I was thinking that I wanted an apple" first person is more like "I want an apple".
Then there's the character and writing. To me it's just words on a page. What am I supposed to feel when he says he forgot everything? Am I happy, sad, angry, frustrated, or what? I don't feel anything when reading your work. And I want to feel. Let us slowly into the protagonist's mind, and make sure to balance your tone. In the passage you said, "Wait a moment, I don't even remember my own name. What was my name? Who am I? What is everything? Who is everyone?". Personally, I would have written, "Hold up. Who am I? Why don't I remember my own name?" and keep going like that, but of course that's only if I want the reader to feel attuned to the character's feelings of anxiety. Personally doing that helps the reader, like me, actually want to know the character and his emotions. You don't want the reader saying something like "He's in pain. He has amnesia. He is about to die. Okay. What next? I don't feel anything". You want the reader to say, "Oh no! He's in pain! What's wrong? What's happening? How did he lose his memories? I wanna read more!". One of the best ways to do this is to give the characters, no matter if they are humans or slimes, humanity. Human emotions, human anything. Just give them something. I find that your character feels less human, less 3-d and more 2-d. The same with your synopsis - you could improve it. "Born into a world, devoid of memories, a boy discovers he was not human. This new, strange world is filled with many dangers. Will he survive?" is good enough. And don't put more than one idea in the same sentence for the synopsis unless it's a list.
You also want to think hard about each and every moment in your book - what happens in the moment? What do you want the reader to feel? How can you get the reader to feel that? For example, my book is a tragedy. So I want the reader to feel sad and horrified. That means I need to put my characters in horrifying contexts and give them slivers of humanity in the middle of all the horrific things. That tends to work. Then the problem is writing it - but there are hundreds of lessons out there on writing with tone and mood so after the thinking the hardest part is done. You have to put the reader in the moment. Put them in there with the characters, seeing what they are seeing, feeling what they are feeling. Vivid verbs, strong words, and sensory language all help with this.
So basically, draw out your sentences and milk each moment for all it's worth. Transition smoothly into the next scene. And use tone and mood. The emotions a reader feels while reading is your very best friend.
But honestly, your idea is solid!
I'm a new writer just like you! My book, titled "
Who We Once Were" is my very first novel. I like to think it serves as a good example of writing with emotion, but I probably am just kidding myself. I also love reading books, far more than I do writing. I have an entire library of books with authors ranging from Jane Austen to Ron Chernow. So those are my credentials.