if they are loose lined, remove and spray lightly with mould and mildew remover ( green bottle with household cleaners at the supermarket).Be careful as this stuff will bleach some fabrics.
if they are sewn in linings, try to pull away from curtain as much as you can and rub with the green household soap available in supermarkets, then rinse just the affected area
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix curtains.