When Othello throws coins at Desdemona, what does he call her?
I suspect you are referring to Act IV, scene ii, when, after striking her in front of Lodovico, Othello finally calls Desdemona a whore. The scene proceeds from Desdemona asking, "[W]hat ignorant sin have I committed?," to Othello finally voicing what he has been jealously fearing for so much of the play:
Was this fair paper, this most goodly book,
Made to write "whore" on?. . .
And when Desdemona denies being a whore, Othello says:
I cry you mercy,
I took you for that cunning whore of Venice
Who married with Othello. You mistress,
That have the office opposite to Saint Peter,
And keeps the gates in hell, ay, you, you, you!
We ha' done our course; there's money for your pains,
I pray you turn the key and keep our counsel.
This "there's money for your pains," I have taken to be the moment you are referring to you in your question. Only, Othello is talking to Emilia, not Desdemona. The "You Mistress" is addressed to Emilia who has been guarding the door since the opening of the scene, which is why Othello makes reference to "Saint Peter" and the "keep[ing] the gates." And the "I pray you turn the key and keep our counsel," means simply, "Open the door now, but don't tell anyone what you've heard here."
So, Othello throws money at a women whom he considers a servant, Emilia, not Desdemona.