Re: DD has a "secret mommy"
I'm so sorry you're having to deal with this. I was be upset as well. I think what bothers me the most about this is your Ex telling your daughter you would be upset about it & making her feel uncomfortable coming to you with this. He is totally messing with an innocent child.
I would be calm and collected when I talked to the Ex about it. Maybe say something about "You would not believe the story DD was telling today..." Then see what the Ex says about it. This lets him know that YOU know without actually accusing him or his GF of anything.