You can't access the app without a passcode, Face ID, Touch ID or Android Fingerprint (on compatible devices). As long as you don’t share your passcode or let anyone else have access to your phone (you may have other people’s fingerprints set up on your phone), your information should be safe in the app.
If you're worried, contact us and we'll stop the app being accessed from the phone. If you think your credit card may have been used fraudulently, contact us straightaway.