1 As that fair star, the messenger of morn,
that fair star > (The morning star, Venus, named after the goddess of love)
2 His dewy face out of the sea does rear: 3 Or as the Cyprian goddess, newly born
Cyprian > (Belonging to Cyprus: Cyprus was one of the chief centres of the worship of Aphrodite (Venus); she is also called Cypria. Thus "cyprian" also = "lewd", "licentious")
4 Of the ocean's fruitful froth, did first appear:
froth > (In later accounts, Aphrodite is said to have been born from the sea-foam: hence her name, from the Greek aphros, foam, froth)
5 Such seemed they, and so their yellow hair 6 Crystalline humour dropped down apace.
humour > moisture apace > swiftly
7 Whom such when Guyon saw, he drew near,
Whom such > [Whom]