Trust this computer keeps popping up

Connect your iPhone, iPad, or iPod touch to your computer or another device. Enter your passcode, if you have one, to unlock your iPhone, iPad, or iPod touch. If you want to allow your computer to access information on your device, select your device in Finder and click Trust, or if you're using iTunes, click Continue.

'Trust This Computer' alert on your iPhone, iPad, or iPod touch

Do you tap to trust it always, or just trust it once?

Trust this computer flashes extremely fast on my phone. I am not able to click the button it is flashing so fast. Have logged out and back in but still keeps flashing when I plug in to my phone.