What does the word mistress mean to the readers? Why does he call her "coy"? Is this poem about seduction? Isn't the speaker asking her to commit an immoral act? What is Marvell's message in this poem?

The "mistress" of the poem is the man's love interest, who he is completely determined to seduce. He calls her coy because she is inexperienced..... she is shy.

As for the immoral act, yes, he is urging his mistress to make love without benefit of marriage, something that at the time would have left her reputation in ruins and brought shame upon her.

Marvel's message seems to be that one should sieze the moment.