It might sound very stupid to read what i will write you now ..... but ..... i had the same problem a lots of time ago and i suggest you to apply a very small blob of a cream cleaner for bathroom (someone use even toothpaste) on the rear camera lens and use a cotton bud to coat the entire lens (or just a cotton), then use a microfiber cloth dipped in water to remove the cream (or toothpast).
If you now take a closer look, you should find that opaque on rear camera glass vanished.
The most important thing to remember here is that you should be gentle when applying and removing the cream (or toothpaste), but apply some force around the corner of the lens (just to see if it works properly) and a lots of patience
I used Cif (it's a cream cleaner for bathroom) (
HERE the datasheet) i don't know how you call in you country, it's like a cream with granules