Do a search for slime creature and you get some interesting results. But what is a slime creature? Hell if I know. Sometimes, when writing stories, you start with a story idea and come up with a name for it as the story develops. Other times, you start with a title and work out from there. So, that brings us back to a slime creature, and not any slime creature, but one from Planet 12. What would Planet 12 be like?
The answer: Planet 12 is a dark place, barely lit by a dim red star. It's warm, it's close to said star in terms of relative distance. It's a swamp-like world. How do I know? Because I said so. One of the hottest things in astronomy right now is the fact they're finding all of these planets. Once they've found them, then you have to figure out what they're like. Then the scientist yields to the science fictionist. who tries to decide what would actually live there. And on Planet 12 there are slime creatures. And that's how we come up with a story idea.
Next, we go from idea to actual story. Are there other creatures beside the slime creature? What's it like on this planet? And you soon have your story.