Bath Mat No Suction Cups How Do You Clean The Black Gunk Off The Bottom Of A Bath Mat?

How do you clean the black gunk off the bottom of a bath mat? - bath mat no suction cups

We use a bath mat for children who fall outside the bathroom. The summit has also been perfect as the day you bought. The fund - the losers - is an almost solid black. Every night is still dry on the tiles. Occasionally, it is washed at 40 degrees in the washing machine - has no effect. Has anyone found a good mixture for cleaning these things?. I am not going to stop and buy new ones. The only thing we need is extra long for 3 children who enter the computer and as such is very expensive. Ideas?