Could I prohibit my ex from having an ex felony around my child in our custody arrangement?

You certainly have a right to try to keep him away from your daugterby moving the Court to amend its orders on custody and placement. But unless the felony committed involved direct harm to children, you'd have to show just how his company harms or could readily harm, your child. (Some people who have been convicted of crimes are actually quite decent people, so you might want to be certain this guy poses a risk, and that you are not just trying to punish your ex-wife.

I would want more details, and it will be up to you to show that the presence of that individual is harmful or dangerous to your children. It all depends on circumstances but generally responsibility for the children is that of the other parent while they are in her possession.