4 Answers

You are leaving out important details so it's pretty difficult to give you an answer.  How old is your daughter?  Who is the woman?  Does your daughter spend a lot of time with her?  Is the woman a care giver?  Is the woman now married to your ex?  Is your daughter calling her "mom" in an innocent way?  In a spiteful way?  Does your daughter see that this upsets you?  Have you spoken directly with your daughter about this situation?  Different factors would indicate different problems/answers.  Give us more info if you want better responses.

Depends on her relationship with the woman.

If I were you, I'd look at what kind of mother I am and figure out why my child feels she needs a different mother. Is your daughter lacking in positive attention from you? When you ask her why she calls this other woman mom, what does she say? Also, your daughter's age may play a factor. If she is young and in daycare, this happens a lot. Children have a hard time distinguishing who or what mom is when it comes to the one who gives them the most care giving. 

Possibly but not so upset than if she calls another man Dad.