If it were my house, I would start with something mild, like a neutral PH cleaner and a soft scrub brush, rinse and dry it well. If that removes it, then you know it was either dirt or mold. If mold spores remain, then it will come back and you will need to try something else. Sometimes improving the ventilation in a bathroom will reduce mold growth. I keep a fan in my bathroom and let it blow the air around after I shower.