The Mariner's first reference to "slimy things" appears in lines 123-126 as he describes the sea creatures within the context of a rotting sea:
The very deep did rot: O Christ!
That ever this should be!
Yea, slimy things did crawl with legs
Upon the slimy sea.
A second reference occurs in lines 238-239 when the Mariner observes:
And a thousand thousand slimy things
Lived on: and so did I.
At the dramatic climax of the Mariner's tale, the mysterious spell is broken; a beautiful moon and "a star or two" appear in the sky, and in the moonlight, the Mariner watches water snakes, no longer slimy creatures:
They moved in tracks of shining white,
And when they reared, the elfish light
Fell off in hoary flakes.
In his continuing description, the Mariner's awe at the sight before him is implied:
I watched their rich attire:
Blue, glossy green, and velvet black,
They coiled and swam; and every track
Was a flash of golden fire.
The water snakes are "happy living things!" Their beauty is so great it defies human description.
The contrast in the Mariner's descriptions can be interpreted in numerous ways. Taken literally, it advances the plot and develops the supernatural elements in Coleridge's poem. When interpreted symbolically, the contrast suggests that the Mariner has repented from his sin of pride in killing the albatross and has returned to grace:
A spring of love gushed from my heart,
And I blessed them unaware.
- - - - - - - - - - - - - - - - - - - - - - - - - - -
The self-same moment I could pray:
And from my neck so free
The albatross fell off, and sank
Like lead into the sea.
Symbolically, it is the Mariner who has changed.