In NV, must noncust parent repay cost of Medicaid received for child by cust parent the way they must repay any cash assistance the cust parent gets?

Nothing in the published case law, or statutes, appears to answer your question.  You should probably ask your ex for specific documentation, and then consult with a family law specialist to find out what, if any legal responsibility you might have for whatever he is talking about.  As Medicaid is a welfare program that does not typically seek to recoup costs, the claim sounds . . . suspicious.