Some people have reported having good luck cleaning their white rubber phone case with hot water and dishwashing liquid.
Others have resorted to:
Nail polish remover (acetone)
Art gum eraser or a pencil eraser
Electronics cleaning wipes
Baking soda and water paste
Citrus orange degreaser
Be sure to test a small unseen area before tackling the whole case or submerging in it in liquid. Do not use bleach. Bleach can break down the rubber.
Remember to check if the manufacturer of your phone case offers a lifetime warranty, which includes discoloration issues. You may be able to exchange it for a new one.