When you created your Postgres cluster (via flyctl) it gave you a username & a password to connect to your database, as well as a postgres://user:password@host/dbname-like URL that you may be using to connect to it from your apps. You can find it there as well.
Unfortunately for security reasons, we cannot access (and thus give you) your Postgres credentials. But again, if you have some app writing data to it, you have these somewhere!
Cheers