I suggest you make an exclusive split with isNew($object) to check if you are doing a New or and Edit action. From here you can process it as desired.
I don't think you can show/hide the password field based on this though, as conditional formatting is based on an enum. Seems a bit much to create a virtual attribute enum for isNew only to show the password field. If you really want this I'd think you are better off with 2 separate forms.