Iam getting white spots on my iPhone 6s back aluminum

Visit an Apple Store or an Apple Authorized Reseller in your region for assistance to diagnose the problem.

And they could offer a possible solution.

You will first have to make an appointment due to the pandemic.

See: Contact Apple for support and service

Contact Apple for support and service - Apple Support

Unfortunately, Apple does not help with cosmetic damage.

Try spraying a few shots of Windex or another cleaner on a very soft cloth and wipe the back of the phone. No guarantees that this will help.