"Strangers" IN Anderson, All One Universe (New York, 1997), pp. 1-21.
Usually, we know the genre of a short story before starting to read it. A story by Poul Anderson can belong to several genres although the book title, cover, blurb etc might make clear that this volume is exclusively sf, fantasy etc.
In All One Universe, Anderson's introduction to "Strangers" states:
"This is 'hard' science fiction, meaning that it assumes nothing a present-day scientist would consider physically impossible." (p. iv)
That helps to orient us when we read in the opening sentence that:
"...a ghost sailed by." (p. 1)
We would have been on our guard in any case. Ghosts do not usually "sail" and we need to learn how the word "ghost" is being applied in this context. Similarly, another hard sf story begins:
"'No dragons are flying -'"
-Poul Anderson, "Outpost of Empire" IN Anderson, Captain Flandry: Defender Of The Terran Empire (New York, 2010), pp. 1-72 AT p. 1.
We soon realize that the first person narrator of "Strangers" is an alien. He has fur, tendrils and a fin. Are the approaching "Night Folk" human beings visiting his planet? At least once before, Anderson began a hard sf novel, The Rebel Worlds, with an alien viewpoint, including the aliens' memories of humanity. "Terminal Quest," also the opening story of a collection, begins with the viewpoint of an alien and "the Strangers" turn out to be human invaders of his planet.