When tubs turn pink it's because of a bacteria that is in the water.
The pink color or pink mold which it's sometimes called is caused by bacteria produced by Serratia marcescens.
You can usually kill the pink mold or pink mildew in your tub or shower by using some white vinegar or even bleach.
The bleach would work much better at killing the pink mold and pink mildew and help it come off the tub or shower much easier.
I use bleach often to clean my tub and shower which helps prevent the pink mold from coming as the bleach also kill the bacteria that causes the pink mold.
The pink mold can be dangerous to your health if left too long so make sure to get rid of the pink mold in your tub or shower.
If you notice any type of mold you need to get rid of it so that it does not make you sick.