I put mine in the washer with a couple of towels. If it’s white use some bleach in it too.
If you mean the rubbery one with the suction cups, I put ours in the washer with a load of towels to provide friction. Also, I add Oxyclean and use warm water. Let it air dry.
If you mean the cloth one you step on when you exit the shower, I just launder it with towels.
I hang my soft plastic matt on the grab bar and spray it with a 10% bleach-water solution on both sides and let it sit, spray again if its cruddy until its clean.
Renee....I also wash mine with towels in machine.