Put some bleach on a old piece of rag wipe around the seal and see how much is removed.(you may be lucky) but if the mould has ate deep into the rubber there is no cure except a replacement seal . I advise you to run a rinse program to clean out the bleach used as can burn clothes .
Basically this has been caused by lots of low temperature washes,40 degrees or less, washing powder that no longer containing bleach ,( non bio due to sensitive skin ) no air flow inside the machine allowing bacteria to thrive by the millions,even smell
Manufactures are now recommending in there books a service wash every month, no clothes added, a bio powder, and a good dose of bleach on the hottest setting 95*.When program has finished pull the soap drawer tray open a bit,and the same with the door to let some air circulate around the interior.
wirecutterman
July 2009
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