Hi
If the marks cannot be removed by cleaning your iPhone in accordance with Apple's cleaning instructions and you are concerned about a potential manufacturing defect, then contact Apple Support (mail-in service may be available, if required), make a Genius Bar reservation or visit an Apple Authorised Service Provider with a view to having your iPhone checked and, if recommended, serviced / replaced under warranty:
- US: iPhone Service Answer Center - Apple Support
- Elsewhere: Apple - Support
More information: