Do I have to open MobilePay on my phone to tap?

No, you don’t need to open MobilePay to tap. On an iPhone, double press the side button, unlock your phone, and tap. On an Android, unlock your phone and tap.

But for this to work, you need to do two things first:

1. Activate tapping (prepare your card for tapping with MobilePay)
2. Set MobilePay as your default payment app in your phone settings

You can find instructions for both in MobilePay.