Go get a new one. Silicone case is available in cheap price. Also, you can try to use a wood case to protect your iPhone. it will look nice just like Oneplus one's rear housing.
If you are talking about the Apple silicone case you can basically clean it with anything - soap and water, leather cleaner, glass cleaner, etc. anything will clean it. Cleans easily.
Last month I ordered silicon case from onx3 for my iPhone. Very soon I realized that it picks a lot of dirt and small stains. I had no idea that it would catch dark spots/marks that easily. I tried rubbing with my finger and nail and it didn't quite work. Then I started searching about how to clean it up. I searched online and got a few interesting tips that I am sharing with you as well.
Here are some tips for your clean your Apple Silicon case:
Use only a soft, lint-free cloth. Avoid abrasive cloths, towels, paper towels, and similar items that might cause damage.
Unplug any external power sources, devices, and cables.