Assalaamu alaykum. if a husband says to his wife, as a way of being romantic, "The moon is not shining because you did not look at it today," or, "Every time you open your eyes, a flower blossoms,' does that fall under Shirk (polytheism)? Is one regarded as sinful for it? He merely does so to express his love.

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

Such statements have nothing to do with Shirk. Yes, there is some exaggeration in it; in Arabic, it is a kind of metaphor. The scholars stated that it is not a lie.

Ibn Qutaybah said in Ta’weel Mushkil Al-Quran:

"When the Arabs wished to express the extent of enormity of the death of a great man who was of high status and great benefit and who did many acts of righteousness, they would say, 'The sun was darkened for his death; and the moon eclipsed for his loss; and the wind, the lightning, the sky, and the Earth wept for his death.' They meant to exaggerate in the description of the calamity that had befallen them, and that it is general, affecting all people. This is not a lie, because they (the speaker and the listener) all agree on this, and the listener (understands) what the speaker intends…”

In addition to this, the Sharia permitted lying between the spouses in matters that bring affection provided that none of them transgresses the right of the other.

Um Kulthoom bint ‘Uqbah, may Allah be pleased with her, said, “I heard the Prophet say, 'He is not a liar who reconciles between people and narrates something good or says something good.' I never heard him give permission to people to lie except in three cases: 'Lying in war, lying in order to reconcile between people, and when a husband speaks to his wife (to please her) and his wife speaks to her husband [to please him].'”

An-Nawawi said, “In regard to the husband lying to his wife and her lying to him: what is meant here is in showing love and promising what is not necessary and the like, but being deceptive in order to prevent what he is obliged to fulfill towards her or taking what he does not possess [has no right to take], or her taking that which she does not possess [has no right in]; then this is forbidden according to the consensus of the Muslim scholars.”

So if a man says statements like those mentioned in the question to his wife, then he is not sinful.

Allah knows best.