Mouse and keyboard prompts show up while I'm using a controller to play Fortnite
If you see your mouse and keyboard prompts while using a controller when you play Fortnite on PC, you can try disabling the On-Screen Keyboard from Windows Settings:
Open Windows Settings
Navigate to Accessibility Settings
Go to Ease of Access (or Accessibility)
Scroll down to Keyboard
Make sure the Use the On-Screen Keyboard option is turned off
If this is already off, try turning it on and then turn it off again.