Is the fiancé's description of his fiancée as "you are like the moon" an ordinary statement, and is someone who describes this statement as ordinary considered a 'dayyuth,' especially if they are one of the fiancée's relatives?

The "dayyuth" (cuckold) is he who condones evil among his womenfolk, and the word that a man would dislike to be said about one of his unmarriageable kin (mahram). His lack of protective jealousy in such matters is a form of "dayyutha" (cuckoldry). It is not permissible for your fiancée to expose herself to temptation by uncovering her face before non-mahram men, nor is it permissible for them to look at her if she uncovers it.