Colloquial. The set phrase
吐沫横飞 (tùmo héngfēi, "spit flies in all directions") describes someone talking with extreme animation — vivid but unflattering.
沫 carries a neutral tone in this word (mo), unlike its standalone reading mò. More clinical synonyms:
唾液 (tuòyè) for medical contexts,
口水 (kǒushuǐ) for general everyday saliva.
