Apple Silicone Case staining the aluminum back?

Hello there, alxtpimenta.

It seems you are seeing some sort of marking on your iPhone after removing the case from your iPhone. The following Knowledge Base article offers up some great information and recommendations:

Cleaning your Apple products

The materials used to make Apple products vary; in some cases, each product might have specific cleaning requirements which vary by the part you're cleaning. To get you started, here are some tips that apply to all products:

• Use only a soft, lint-free cloth. Avoid abrasive cloths, towels, paper towels, and similar items that might cause damage.
• Unplug all external power sources, devices, and cables.
• Keep liquids away from the product.
• Don't get moisture into any openings.
• Don't use aerosol sprays, solvents, or abrasives.
• Don't spray cleaners directly onto the item.

If liquid does make its way inside your Apple product, get help from an Apple Authorized Service Provider or Apple Retail Store as soon as possible. Liquid damage isn't covered under the Apple product warranty or AppleCare Protection Plans. If you plan to visit an Apple Retail store, make a reservation at the Genius Bar (available only in some countries).

Learn how to disinfect the Apple internal or external keyboard, trackpad, and mouse.

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iPhone

Find your iPhone model below.

iPhone 5s, iPhone 6, iPhone 6 Plus, iPhone 6s, iPhone 6s Plus

Clean iPhone immediately if it comes into contact with contaminants that might cause stains, such as ink, dyes, makeup, dirt, food, oils, and lotions. To clean iPhone, unplug all cables and turn it off. Use a soft, slightly damp, lint-free cloth. Avoid getting moisture in openings. Don't use window cleaners, household cleaners, compressed air, aerosol sprays, solvents, ammonia, or abrasives to clean iPhone. The front glass surfaces have an oleophobic coating. To remove fingerprints, wipe these surfaces with a soft, lint-free cloth. The ability of this coating to repel oil will diminish over time with normal use, and rubbing the screen with an abrasive material will further diminish its effect and might scratch the glass.

To clean the Home button (allowing Touch ID to work optimally), wipe it with a soft lint-free cloth.

iPhone 5c

Clean iPhone immediately if it comes into contact with contaminants that might cause stains, such as ink, dyes, makeup, dirt, food, oils, and lotions. To clean iPhone, unplug all cables and turn it off. Use a soft, slightly damp, lint-free cloth. Avoid getting moisture in openings. Don't use window cleaners, household cleaners, compressed air, aerosol sprays, solvents, ammonia, abrasives, or cleaners containing hydrogen peroxide to clean iPhone. The front glass surfaces have an oleophobic coating. To remove fingerprints, wipe these surfaces with a soft, lint-free cloth. The ability of this coating to repel oil will diminish over time with normal use, and rubbing the screen with an abrasive material will further diminish its effect and might scratch the glass.

To clean the dirt that might accumulate around the SIM tray opening, use a soft lint-free cloth and place a very small amount of rubbing alcohol on it. Very gently wipe the dirt away from the port opening and avoid getting any of the moisture in the opening.

iPhone 5

Clean iPhone immediately if it comes into contact with contaminants that might cause stains, such as ink, dyes, makeup, dirt, food, oils, and lotions. To clean iPhone, unplug all cables and turn it off. Use a soft, slightly damp, lint-free cloth. Avoid getting moisture in openings. Don't use window cleaners, household cleaners, compressed air, aerosol sprays, solvents, ammonia, abrasives, or cleaners containing hydrogen peroxide to clean iPhone. The front glass surfaces have an oleophobic coating. To remove fingerprints, wipe these surfaces with a soft, lint-free cloth. The ability of this coating to repel oil will diminish over time with normal use, and rubbing the screen with an abrasive material will further diminish its effect and might scratch the glass.

iPhone 4 and iPhone 4s

Clean iPhone immediately if it comes into contact with contaminants that might cause stains, such as ink, dyes, makeup, dirt, food, oils, and lotions. To clean iPhone, unplug all cables and turn off iPhone (press and hold the Sleep/Wake button, and then slide the onscreen slider). Use a soft, slightly damp, lint-free cloth. Avoid getting moisture in openings. Don't use window cleaners, household cleaners, compressed air, aerosol sprays, solvents, ammonia, abrasives, or cleaners containing hydrogen peroxide to clean iPhone. The front and back glass surfaces have an oleophobic coating. To remove fingerprints, wipe these surfaces with a soft, lint-free cloth. The ability of this coating to repel oil will diminish over time with normal use, and rubbing the screen with an abrasive material will further diminish its effect and might scratch the glass.

iPhone 3GS

Clean iPhone immediately if it comes into contact with contaminants that might cause stains, such as ink, dyes, makeup, dirt, food, oils, and lotions. To clean iPhone, unplug all cables and turn off iPhone (press and hold the Sleep/Wake button, and then slide the onscreen slider). Use a soft, slightly damp, lint-free cloth. Avoid getting moisture in openings. Don't use window cleaners, household cleaners, compressed air, aerosol sprays, solvents, ammonia, abrasives, or cleaners containing hydrogen peroxide to clean iPhone. The front glass surface has an oleophobic coating. To remove fingerprints, wipe these surfaces with a soft, lint-free cloth. The ability of this coating to repel oil will diminish over time with normal use, and rubbing the screen with an abrasive material will further diminish its effect and might scratch the glass.

iPhone 3G and 2G (Original)

To clean iPhone, unplug all cables and turn off iPhone (press and hold the Sleep/Wake button, and then slide the onscreen slider). Then use a soft, slightly damp, lint-free cloth. Avoid getting moisture in openings. Don't use window cleaners, household cleaners, compressed air, aerosol sprays, solvents, ammonia, abrasives, or cleaners containing hydrogen peroxide to clean iPhone.

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Thanks for reaching out to Apple Support Communities.

Kind Regards.

Are you sure something (some type of fluid/cleaner/etc) didn't get on the iPhone or in the case while you were cleaning it? That's the only thing I can see as being able to discolor the aluminum in one week's time (between your normal cleaning).