What if your goldfish seems to be trying to mate with your black Mollie is that possible?

Goldfish are cold water fish and should not be kept in an aquarium with a tropical molly. There is no way they can cross breed. Fish are not stupid and do know the difference between one of their own species and a species that should not be in there with them. It is quite possible the goldfish will eventually kill the molly. Failing that, if your tank is not heated, the Molly will probably die during the winter if the temperature gets below 65F for too long anyway.