My little kids have a glorious time playing in mud puddles when we get rain. I can’t get the grey mud stains out. I have been rinsing the wet muddy clothes, spray with Vanish Gold stain spray and wash in Biozet powder.
284900cookie-checkmud stains on kids clothes
It probably just needs mechanical agitation. It’s a physical stain. The grains are physically embedded into the fabric and will resist most chemical approaches. Squirt any liquid detergent on, rub in with with your fingers and rinse. Or a steam cleaner will also work.