Any solutions or products for getting molded tub surface clean

[Victoria Allen]

Hello Bigger Pockets landlords and flippers,

Does anyone have a solution or products to recommend to get a molded tub floor clean? 

It looks like a dirt colored substance was ground into the textured floor surface but it doesn't wash off. I have seen this in toilets also. Common household cleaners don't work. I don't want to yellow the tub with bleach even if it did work.

Thanks for any helpful ideas that doesn't have me replacing the whole tub.