Use Chlorox bleach, slightly diluted, you can add some laundry
detergent to it. Let it do its job for half an hour and then rinse.

If the discoloration persists, you can also use 10% hydrogen peroxide
(not stronger! use gloves!) but be aware that silicone sealant putty
around the bath may get attacked.

To keep the assorted fauna and flora from re-growing, it helps if you
clean your mossy bathtub corner on regular basis. I do it too, at least
once a year - whether my bathroom needs it or not. (But the easier
alternative is to move into another apartment.)