Question about which kanji should be used.

[Achilles 1-1]

So demon slayer has a thing called Mist breathing 霞の呼吸 Kasumi no kokyū however, doesn't the kanji "霞" mean "Haze" and the kanji "霧" "kiri" mean mist? Which one should be used?

 

[Majestic]

霞 is fine for "mist".
霧 (kiri) is "fog".

Although, there is a super fine line between mist and fog, and in certain contexts they can be interchangeable. You can't always get an exact one-for-one translation of words from one language into another.