It is residue or hard water deposits of the last user. You can try a hot load with 3 cups of bleach in it, or use washer magic(special cleaner for washers good for hard water areas). When you use these to clean it be sure to add 2 gallons of water to it after it has stopped filling. This will raise the water level above the ring on the outer tub and not over flow it. I have included a video of how to remove the cabinet if you want to get in there and clean it manually. All you have to do after the cabinet is removed is pop off the splash ring and you can get in there with a brush.
Washer magic click here