"Apparition" (line 1)
"Apparition" has traditionally been used to describe ghosts or phantoms, but it can also just refer to a sudden appearance of any kind. Now you see it, now you don’t. Pound takes advantage of these two uses of the word to confuse the reader into thinking that the faces in the subway might be ghost-like. J.K. Rowling does the same thing in the Harry Potter series by saying that characters can "Apparate." We know that Harry and his friends aren’t really ghosts, but the word lends them an aura of the supernatural.
"Petals on a wet, black bough." (line 2)
The second line is notable because it doesn’t contain any reference to the supernatural. We might have expected the poem to follow up on the hint of ghosts provided by the word "apparition," but instead we get a short but really specific image of the natural world. On the other hand, this is the world of the poet’s imagination, so in this sense it could be considered "above or beyond nature," which is the literal meaning of the world "supernatural." (Cool fact: the word "super" actually means "above" or "beyond," which makes sense, because we all know that "superheroes" are way better than normal heroes.)