That pop up means you stored your credentials for that site in safari's keychain. Upon returning to the domain it found a match and is asking for permission to auto fill your credentials to log you in. If you accept, it should not pop up anymore. You can check to see if the information is correct by going up to safari's preferences and finding the tab that stores your passwords and checking the box at the bottom to show your password. It will ask for your admin password to show you the saved password.