If you have to add a translator note to explain what a word means, then you should just translate the word. Seiso = pure. Then just write pure in the actual speech bubble!
And I don't think seiso is a commonly understood word in the English-reading Manga community. It isn't like senpai or kawaii where it's generally understood. Maybe I'm completely wrong and people do already know it, but it just looked really awkward when you could translate the singular word seiso into the single word pure. There's no point in not translating.
Sorry if I sound ungrateful for the translation. I am not. I do appreciate the work of the translators. I'm just saying that looked really awkward to put that note like that.