Have you tried scrubbing with a Magic Eraser? It was the only product that worked on one of our bathtubs. It also had gray marks, but who knows what from.

What is your bathtub made of? If it is porcelain, Barkeeper's Friend will work.

Not sure what tub is made of, but I will give those ideas a try. Thank you!

Be careful if it's acrylic and not some kind of metal. Try a magnet if you don't know how to tell. Do not use magic eraser on acrylic or fiberglass. Or Comet/Ajax type cleaners. I would try a paste of water and baking soda first. It should work. If not, as Nancy says, Barkeepers Friend either liquid or powder should work safely, too.

Good tip on the erasers. I had no idea it could harm some finishes. I used it on porcelain.