This is definitely one of the less looked upon issues in stories, as authors can usually figure out how they want to do it in their stories, but when done wrong, it can ruin a great idea. This is not to say that it is extremely important, but prose can really drive home the point of a story if used correctly. It can be used so effectively as to make a story scarier than it could have been had the author not considered the prose in the first place.
What is prose?
Prose is basically your "author's voice". It is the vocabulary you use when writing a story. For me, this is written in the prose I'm most comfortable with, and for you, starting out, your first story should be too. If you are a more seasoned author, you can experiment a lot more with the nature of your prose, but try to get comfortable with what you know best - your own.
Prose and Mood
Everyone has a different way of writing, and so no two people's prose will be exactly alike. For example, look at... say, Doom Vroom's style in Closet Monster, vs. Squidmanescape's style in They Took My Soul from Me. Don't worry, I'll give you a minute. I've got the time.
So, in Squid's story, the style utilises very... mysterious wordplay. It uses few enough words that the story can progress, but still controls your speed by using enough to make the story progress slower (and all without ellipses! Fancy that!). It also uses a lot fewer adjectives than that of a more 'normal' story, leaving the reader's imagination to fill in most of the gaps.
Now, in Doom Vroom's story, it uses a much different approach. The story's prose is less mysterious, which helps to tell the reader how Doom imagined the monster, while their partial lack of completely in-depth descriptions lets the reader fill in the blanks. This is extremely useful in portraying more realistic fiction, though it can be used in certain scenarios where the author wants things to happen quickly.
LOOK AT MY BIG WORDS!
Greetings, females of the species homo sapiens, along with their male counterparts! My colleagues and I are at this most wonderful encampment on the date of March Fourteenth, 1972, to bring to all of those that are viewing this a most magnificent show.
That is an example of overdoing it. Just because you know 'big words', does not mean that you have to use them. It may help in certain situations, like when you are writing and don't want to repeat a word for fear of it sounding awkward, but overdoing it just to impress people will not work. One of the best examples of this would probably be Umbrello's reading of Snowy Dillon. Watch it. It's exigent.
All jokes aside, pay attention to the way the author overuses the word 'exigent'. Even the first time, it sounds out-of-place, and after the third or fourth use, it begins to sound beat-up and tired. Now, apply this to your own writing. Only use words that you are familiar with, or that the prose would use.
Using words like this is fine, as long as it fits into the author's voice well. Don't use words just because they're big.
So, about the big words thing, I may have lied to you a little bit. In the Snowy Dillon example, I told you about how using words that don't at all fit with the prose is evil and should never be done ever not at all? Well, there are some situations where that is not true. Say, for example, I was writing a journal, and supposedly the narrator was writing the journal for their teacher. There is somewhere it would be acceptable to use 'big words', as the kid would be trying to impress their teachers. Another example would probably be a science story, and perhaps some real science words are used in there (critical, conditional, nucleus, fission, etc.)
Anyway, if you're not writing one of these, forget you ever saw this. I hear they have something for that over at the FBI headquarters. Let's just keep the fact that I know about that between you and me, okay?
How can I change the prose?
My best answer is probably to get inside the head of your narrator. You can do this in a lot of ways, perhaps by immersing yourself in the story you are writing (If you can immerse yourself, it's not much different than reading a book). This will help to do two things: First, it will help you create believable characters and scenarios, and secondly, it will help you to create a prose based around the character.
When doing this, (or even if you can't and just want to write,) ask yourself some questions. Where is the narrator from, and what's the language like there as opposed to here? How old is the narrator (this is especially important in journal pastas)? What is my narrator feeling (this is easier if you are immersed in the story)?
Those questions should help guide you through the story without too much hesitation when it comes to prose. If you do find it tough, try asking these questions again, and perhaps rereading your story up to the point you're at. That should give you a sense of the voice in this story, and perhaps, if you've used it before, the term you were looking for.
Why should I know all this anyway?
Simple. Prose can make or break a story. If done correctly, it can enhance the mood, the feeling of fear, and even how vivid the story's descriptions are to the reader. Perhaps they are in a state of unfeelingness, like in They Took My Soul from Me. A prose that is moderately descriptive would ruin that by not allowing the reader enough room to gaze into the words and figure out what they mean for themselves would ruin it, but can enhance a story like Closet Monster. Each story's prose will be unique to it, and each writer's prose is unique to them, just like a fingerprint, so use yours to your advantage to create the best stories you can.
Hope this helped.