White foamy cast on back camera lens

Can you take a picture and show us what you see? Generally speaking, since we can't see what you see, if you can't wipe it from the camera cover, then you should schedule an appointment at your Apple Store Genius Bar and show it to Apple in-person --> Genius Bar Reservation and Apple Support Options - Apple

From what it sounds like, it may also be moisture that got caught in the camera lens. But whatever it is, it's likely a hardware or physical issue that as lobsterghost1 states, needs to be taken a look at your nearest Apple Store's Genius Bar or Apple Authorized Service Provider.