soap residue on laundry is generally too much soap or overloading with laundry. As for mould its unsightly but not over troublesome if the seal is clean on the surface. I’ve seen lots of ideas bleach, milton fluid, vinegar, brick acid, etc, etc, but if the customer is worried ……..new seal. I did read some time back about someone coming up with an anti microbial rubber what became of this I dont know. Other than that start with new clean seal boil wash regularly and keep dry after use.
wilf