In ghadhāhā, ghadhā is a perfect verb and the attached pronoun -hā is its object. Namīru al-māʾ follows later as the grammatical subject: pure, pleasant water is represented as having nourished the referent.
Namīr describes agreeable, fresh water. Ghayru al-muḥallal excludes undesirable mixture or alteration. The water becomes a symbol of clean nourishment and preservation, not just a beverage.
The construction shows that an Arabic subject need not immediately follow its verb. Confusing the object pronoun -hā with the subject reverses the relationship expressed in the line.