An interesting take!
It’s quite close to the original story/prompt… The explanation that it was just a nightmare adds a nice little twist.
But what if there was another angle?
Imagine if Susy wasn’t real at all, just a doll that kept haunting her dreams. Or maybe Susy existed once, but only as a memory, and what she sees beside her is the shadow of her own childhood self. That would give the story an even darker, more mysterious edge. ;)))
Greetings my friend. If the truth is that you can also give an interesting twist to the story through there. You could even consider that Susy is an imaginary childhood crumb who has returned after a strong psychological problem. The options are multiple my friend, however in the end I decided to explore something like a common phobia (sometimes it's surprising how many people we end up knowing who are afraid of thunder) and build the short story from the psychological terror that it can generate to the sufferer of the phobia.
Although being very honest, I confess that I have written very few lines in the horror/horror genres.
You have done an excellent job!