Out of envy for the beloved's clear and radiant face, water (tears) has welled up in the mirror's eye.

The poet brings the mirror to life with a lovely touch of personification, imagining it as an onlooker capable of feeling envy. The beloved's face is so luminous and flawless that the mirror itself feels outmatched, and tears well up in its 'eyes' out of sheer jealousy. It is a charming way of saying that the beloved's radiant beauty outshines the very object whose only purpose is to reflect clarity.