There are so many ways to clean a phone case. And here you can see some of them
Firstly you can try to clean your phone case with the help of dish soap.
The another way is to clean you phone case is disinfecting your phonecase with rubbing alcohol.
You can also try the toothpaste to clean your phone case.
Using the paint thinner is also one of the way to clean the phone case.
If your phone case is not cleaned after tried all of these ways. Then maybe it's time to buy a new phone case.