Plastic trim is porous and sometimes difficult to remove. The white discoloration you are seeing is the clay that is in all car polishes. Use mineral spirits for removing the white residue left behind on plastic trim. Car polishes and waxes contain a cleaner that will leave a haze on plastic trim.You may need to use a tooth brush or an applicator like a tooth brush that can get into the deep pores in the plastic. It could take more than one application, especially if the polish was applied some time ago due to the exposure to ultraviolet rays.