The wick filter may develop a white or yellow buildup after some use. Mineral deposits and other debris, that originated in the water used to fill the humidifier, that have been trapped by the wick filter cause this buildup and staining.
To clean, remove the wick filter from the filter cage and rinse under running water. For a deeper clean, soak with mild or citric acid cleanser, 20g citric acid for every 1L of water for 1 hour, then rinse well under running tap water according to the instructions in the user manual.