It is important to realise how these "slimy things" compare to the "water snakes" in this poem. Remember that the "slimy things" appear when the death of the Albatross begins to be avenged by the Polar Spirit, the force of Nature who punishes the Mariner and the whole ship so vindictively for the murder of the albatross. It is in Part II when we are introduced to the "slimy things" who come along with a deadly calm in the wind and the sea:
The very deep did rot: O Christ!
That ever this should be!
Yea, slimy things did crawl with legs
Upon the slimy sea.
The profound stagnation that leaves the ship abandoned without any hope of movement or relief creates such a decay in the sea that unnameable monsters are said to "crawl with legs" upon the slimy sea. Clearly this is an abhorrence of nature, designed to punish the abhorrent act against nature--the murder of the albatross.
However, these are contrasted with the beauty of nature as symbolised by the water snakes in Part IV. Compares to the "slimy things," they are described in terms that communicate their beauty:
Beyond the shadow of the ship,
I watched the water snakes:
They moved in tracks of shining white,
And when they reared, the elfish light
Fell off in hoary flakes.
Within the shadow of the ship
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 contrast is hopefully clear. Seeing these creatures causes the Mariner to, almost involuntarily it seems, bless them, and thus break the spell that is upon him. Whereas the "slimy things" were a symbol of his damnation, the water snakes symbolise his redemption and release.