Me and my husband are separated, not legally separated in U.S. I have the papers for marriage in U.S. we have been separated for 6 years in U.S. I recently found out he married someone else in panama, is this valid, we are not divorce in U.S.

Unless he divorced you somewhere, without your knowledge, no it would not be a valid marriage in the US, and most other parts of the world that do not allow more than one wife.

Most likely he merely filed that he was divorced and lied on the marriage application