Greetings, @d-pend
I don't know if you drink; my guess is you don't, but it occurred to me that amid these late changes in your life you might be lucubrating while drinking a shot or two, on the rocks. The images made me think of ice, ice cubes melting in a glass. With all the distortions and blurred vision that implies.
Who said crystal is always clear? People like to refer to honesty in those terms.
Neither is truth, or our knowledge of anything.
Maybe that’s the “humes” alluded in the first stanza. David Hume, who rejected the possibility of certainty in knowledge.
The invocation to the water nymph—neglected, forgotten—to emerge from darkness; from the enclosure of the body, the limitations of earthly life.
The poem suggest that whatever truth we come up with, whatever certainty we presume of becomes nothing but a distortion of higher powers, of higher truths.

Astroturbid rays of mine:
away, allay a blur divine

A final lines sound like an invocation to the primeval state, the idyllic, pastoral transparency of man in direct connection to the cosmos, if ever was. “No crystal truth can stay”, a fragile affirmation, either because it can break or because it can be tainted, blurred or even melted.