I love the analogy woven into this. While the mermaid has her tale in the light of day, everything is bright and vivid. Your word chocie is beautiful, making everything feel so vibrantly alive, like freedom. Then, when dusk calls, and she's forced to leave her tail behind, the mood changes drastically. Everything slows, and the word choice and imagery brings the macabre on so suddenly it's almost shocking.
While there are a lot of different analogies I can think of, the first that comes to mind is the circle of life, where when the mermaid has her tail, it is representative of youth. She is full of life and energy as she dives just as deep as she can, the sea is hers. It reminds me of being young and free and having the feeling that you're unstoppable, like the world is yours. Then as the sun sets, it reminds me of aging. The mermaid retires her fins (being unable to swim freely in the ocean's depths) in the same manner that the elderly can no longer do the things they once could.
The loss of light and decent into night (or darkness) could be representative of life and death in this instance (light representing life and darkness representing death), and the way the sun is setting ("the sun bleeds low, burnt orange, no longer pure") screams of the process of aging and dying, especially when you take into account that youth generally represents innocence and the image of blood, in terms of death and dying.
Of course, then there's the more concrete evidence with the dying light, which of course, reminds me of death, as do the bodies growing algae (which reminds me of old things, because old things are known for growing things like mold -- though that one might be a bit of stretch).
At any rate, I loved this (in both a concrete and abstract sense).
[Quadpot | Match Three -- review a story with no reviews.]