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?. . .

Impudent
strumpet!

And when Desdemona
denies being a whore, Othello says:

readability="22">

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.