I hate to be blunt but this feels unfinished. Additionally the 'you are next' idea has made it onto our cliche list for not really being a strong way to end a story. Lines like this: "That is the thing killing all of you, the thing that kills you in all your dreams is one person." would do a whole lot better if they were actually fleshed out and built up the inherent terror of the situation rather than just the bare bones synopsis of what's happening (it's like pulling up a statistic of people dying in their sleep and not building on the idea and creating anything interesting/involving from it). As of now, leaving this story as it is feels more like you accidentally published a premise rather than an actual story.
Additionally I recall reading a more fleshed out version of this idea for Dreams That Can Kill. Even I've toyed with the idea in I Was Fourteen When I First Killed Myself. I can look up a few more, but I think pointing you in the direction of the dreams/sleep category would be more beneficial as it would help give you some focus on what tropes to avoid (ones that have been overused and don't really add anything new to the idea.)
Here's a more in-depth guide on writing flash-fiction. I would suggest reading it because as this stands, this feels like a starting point and not an actual involving story. Writing flash fiction/short fiction is hard so I would recommend actually writing out a fully fleshed out story before trying to focus on something more difficult.