Contact your lawyer and have the custody agreement opened back up (there is a custody agreement, right?), and tell them what you told us.
If there is no such agreement, sue for one. It seems to me that you are looking for full custody. That would neatly take care of the perceived issues.
As to who to call in the meantime, I'd go with CPS. But understand that some of what you are saying here might not be provable, or even relevant. The amount of time this guy served in jail is not relevant. What you "kind of" know about the living situation isn't enough. It's not proof; it's speculation.
Concentrate on what you can prove, e. g. that the little girl seems unhappy to return to the mother's home, and that the mother is a felon (that's a matter of public record and, since it is, I wonder why the mother has even shared custody. That would be a big thing in deciding custody, which makes me think this is an informal arrangement).
Make it a formal arrangement and sue for full custody (and visitation for the mother, at a time and place of your choosing).
This means a lawyer. It may mean having the child testify, BTW. But that's the path.