There may have a build-up of oils from insufficient cleaning, storing in the refrigerator, or washing in the dishwasher. Fortunately, the silicone is medical grade without any pores that can harbor bacteria, so any film or bacteria is only on the surface.
First, try scrubbing the film off with a bristle brush and a detergent like Dawn Original (yellow ducks on the label).
If that doesn't work,
Reviewed by Diana West, IBCLC
June 10, 2021