I would hardly worry about it. The peculiar (and sometimes illogical) use of articles is one of the most obscure and difficult parts of English grammar. I learn about it just trying to put it into words.

"Where, among the swampy water lilies,/fog lies like smoke" and "Where, among the swampy water lilies,/a fog lies like smoke" are both correct grammatically. In fact, they mean almost exactly the same thing. The technical reason is dishearteningly specific. "Fog" is frequently (though not always!) used as a mass noun, and when it is used in that sense it is grammatically incorrect to use an indefinite article with it. Mass nouns are not plural nouns, but the indefinite article cannot be applied to them in the same way that it cannot be applied to a plural noun. To illustrate the difference between mass and plural nouns - "Fog causes car crashes." (fog here is a mass noun) is grammatically correct while "Fog cause car crashes" (fog here is a mass noun used in the same way as a plural) is not grammatically correct, leaving the subject "fog" and the verb "cause" in disagreement. However, since "fog" is not always used as a mass noun, the sentence "Fogs cause car crashes." ("fogs" here is a true plural, so the verb conjugates differently) is possible, and correct. So the difference between the examples "Where, among the swampy water lilies,/fog lies like smoke" and "Where, among the swampy water lilies,/a fog lies like smoke" is that in the first translation the word "fog" is used as a mass noun, and evokes in the reader/listener a general image of fog, and the types of fogs that hang over witches' huts, while in the second it is used to refer to the specific fog that hangs over that witch's hut, but that the listener/reader is presumed to not be familiar with. The sentence "Where, among the swampy water lilies,/the fog lies like smoke." is also possible, but it assumes that the reader/listener is familiar with the fog that the speaker is talking about.

Incidentally, in English we often use the verb "hang" with the noun "fog". Fog hangs over things. I didn't think of that until writing this. I strongly suspect that the verb "hang" can only be used with the word "fog" where the preposition "over" is used, because the translation "Fog hangs like smoke" (i.e. where there is no preposition) sounds incorrect. I have seen the words "hang" and "smoke" used in the same way. "The smoke from the wildfire was hanging over the city."

This all holds true for the word "fog". The word "smoke" is more truly a mass noun, and instances of "smoke" being used as an overt plural, rather than as a mass noun, are very few. So the sentence "Smoke causes car crashes" is equivalent to, and as correct as, the sentence "Fog causes car crashes"; but while "Fogs cause car crashes" is possible, "Smokes cause car crashes" is not.

(AS AN ASIDE) Despite it being possible, "fog" is not very commonly used as a plural, and in most situations the mass noun form will suffice. So if you were to translate "Там, за туманами" into English, you would say "There, behind the fog", rather than literally as "There, behind the fogs". The article "the" is used here because the speaker is talking about a specific fog that is apparent to both the speaker and the listener, shown by the word "там", "there". Both the speaker and the listener are standing on a boat, looking into the fog; the fog is known to both, so it is described by the definite article. He's not talking about any fog, but rather about that fog over there.

There are situations where it is possible for the word "a" to be correctly applied to smoke, "A smoke was pouring out of the apartment window", but the sentence without the article, "Smoke was pouring out of the apartment window" is just as correct. The safest bet with the word smoke is to never apply an indefinite article to it, since the word "smoke" is not uncommonly used to mean the same thing as "cigarette", and this meaning is almost always when you'll see the indefinite article attached, or the word smoke as a true plural. "Can I have a smoke(cigarette)?" "I only have three smokes(cigarettes) left."

The most detailed discussion of articles in English I am aware of, and where they are used, can be found in this book: https://u1lib.org/book/2663577/6d9251 between pages 367-378. It's a pretty serious grammar book, however, and it would be difficult to read even for most native speakers. I imagine if you can read that book well enough to understand when English speakers use definite and indefinite articles, you already speak English well enough to not need the book too much.