You have dead pixels. If your iPhone is still in warranty or under warranty with Apple Care Apple will replace the screen. If your device is not under warranty, contact Apple Support or an Apple Authorized Service Provider for assistance.
You need to have Apple perform diagnostics on your iPhone. It can be done in person at an Apple Retail Store or at an Apple Authorized Service Provider. You can also contact Apple Support and request remote diagnostics.
If you’re in the US please call 1-800-MY-APPLE or outside the US, consult the support articles below.
Contact Apple for support and service - Apple Support
Genius Bar Reservation and Apple Support Options - Apple
Explore Service Locations
Official Apple Support