This is usually a build up of undisolved laundry dirt which manifests itself as mould and algae. Clean the door seal with an antibacterial cleaner.
Important
In the interest of consumer safety, any adjustments, alterations or replacement
parts should only be made or fitted by a suitably qualified and competent person
with the machine disconnected from the mains supply.
To help keep it clean, wipe the door seal dry after every use, and once a month do a maintenance wash at 60°C or above with a detergent that has a laundry bleach in it, and with fabrics that can take that temperature. As a guide colour care, gentle and woollen detergents do not have bleach in them.
Important
In the interest of consumer safety, any adjustments, alterations or replacement
parts should only be made or fitted by a suitably qualified and competent person
with the machine disconnected from the mains supply.