Algae Colour teeth stains. Cause stains?

Hello everyone, Just a month ago my teeth scaling was done because there were black stains on the lower teeth and partially on the upper teeth where the gum lining ends on the teeth. Again after only one month there are algae Colour stains on the lower teeth and upper teeth as well. On the upper teeth those stains are exactly seen where the gum ends and teeth starts. If i scratch it Using nails, it lightens but it is spreading to each tooth. What may be the cause of the stains? Eating seacod fish oil capsules will stain the teeth? I will take 2 capsules daily. And I even take 200ml of full stack protein shake after my workout session. Will the vitamins and minerals in the protein cause stains? Please help.