Ghayru is a construct noun expressing exclusion or difference, and al-muḥallal is its genitive complement. In this verse the expression concerns the quality and purity of water, not the fiqh categories of lawful and unlawful consumption.
The commentary stresses the water’s pleasant, unspoiled quality. The contextual translation therefore concerns clean or unadulterated water, rather than a legal ruling about drinks.
The imagery has two linked stages: first soft, bright coloration, then wholesome nourishment. The second stage reinforces the first by suggesting an unspoiled origin.