Water-insoluble compounds have ended up on the laundry. Detergents sometimes contain compounds (zeolites) to aid water softening which are not soluble in water.
Brush the residues off the laundry with a soft brush once it is dry.
Use a detergent without any zeolites to wash dark fabrics. Liquid detergents do not usually contain zeolites.
Use the “Dark garments” programme to wash dark fabrics.
