If you're seeing a message each time you launch your game that isn't familiar, it is likely your Apple Game Center nickname, which can easily be changed within your device settings.

 

To update your nickname:
1. Open your Apple Settings
2. Scroll down and select Game Center
3. On the next screen you will be able to make changes where it says Nickname. You can also make changes to your player avatar and update your profile privacy settings

To turn Game Center off:
1. Open your Apple Settings
2. Scroll down to select Game Center
3. At the bottom of this page, scroll down and tap Sign Out (Please note: by turning off Game Center, your statistic and scoring information will not be saved and cannot be transferred to a new device)

If you would like to turn Game Center notifications off:
1. Go to your Apple Settings
2. Select Notifications
3. Scroll down and select Game Center
4. Toggle off Allow Notifications