I cast a null vote because I could see room for improvement but, at the same time, I didn't dislike the poem.
1.) Consistent use of grammar. You use a period at the end, but there are clearly other sentence breaks in the poem.
Strangely, I liked the poem more when I read it every other line, wrapping around back to the beginning to catch the even lines, eg.:
Twirling clouds evade
a whisper of a dream,
a moment in time
elusive.
A captured thought comes forth,
muddling up from depths unknown.
Who can tell what it sees?
Then [it] slips away [un]mourned.
Unyielding.
Biting the will of knowledge.
It is hard for me to say why I like it better that way. Perhaps it makes more sense, or is more cynical (I like cynicism), or just has better subjective flow. Still, this just shows there are many ways to read poetry, and not necessarily in the order the words are written.
Nice job. It was, after all, evocative.