Fill bathtub with hot water while adding a grease-cutting soap like a good dishwashing detergent.
Wash the foam well, squeeze out foam and drain soapy water from tub. Add new water and rinse foam. You may have to rinse several times. Use water as hot as you can stand. Cuts the vestiges of the grease/oil a little better.
Make sure the foam is bone dry before you put it back in the case. Airing outside on a good, hot sunny day is the best ticket.