It’s probably iron mould, most likely from a stray coin or screw left in a pocket that got trapped between the drums.
Check the drum holes; if you think there’s something inside that you can’t get to, you’ll need to call an engineer.
It could also be iron in the water from roadworks nearby. Check by running tap water through a white cloth for a few minutes; if there’s iron, you’ll see it.
To treat, wash as normal or if stubborn rub with lemon juice and salt, leave a few hours then rinse and wash as normal.
Iron mould spreads between clothes, so treat straight away.