If the discolouration is as shown in this picture then the problem could be mildew.

If the discolouration is mildew then you remove the mildew with an appropriate product.

Removal of mildew is an occupier's responsibility under the terms of their tenancy agreement.

Prevention is better than a cure, so you should take care to wipe away excess water and also to adequately ventilate the area (open a window or leave the extractor fan running) after use.

For tips how to remove mildew please watch the following clip: