We have a garden tub in our master bath. In the past 2 months we are getting a black ring around the tub as it is filling up. It is very difficult to get this substance off the tub. Does anyone know what this is and how to fix it?

We have a garden tub in our master bath. In the past 2 months we are getting a black ring around the tub as it is filling up. It is very difficult to get this substance off of the tub. Does anyone know what this is and how to fix it?
Though I do not know the reason for the ring, you might try a Mr. Clean magic eraser sponge. They work wonders.