Helmet headband is getting wet from sweat
Most helmets (certainly the last 6 or 7 I've owned) have the pads held in by velcro so can be removed and washed or replaced. A lot of people I know wear either a headband or a "buff" (google it, it's a brand name) to help a bit with the sweat.

All that said, I'm surprised your helmet pads don't dry out. I also live in Luxembourg and I can ride morning, noon and evening and each time come back to a mostly dry set of helmet pads.