This is probably a mould which is easily dealt with by wiping with a strong bleach solution, then running the machine at the hottest setting, with no washing, and using a biological powder (it must be powder) and no conditioner.
It is becoming more common as we wash on lower temperatures with non-biological washing detergents. There is nothing to kill bacteria which thrives in the warm, damp atmosphere.
All washing machine manufacturers now recommend running this 'empty cycle' once a month. I buy the cheapest bio powder I can find for just this purpose.