My partner swims in an indoor heated chlorinated pool most days. Her swimming cap (pictured) lasted about a year before ripping, but over that time it developed these hard brown growths. They cannot be rubbed or scratched off, and seem to be part of the cap material in some way. What are they? The cap is 100% silicone, was washed with liquid soap every day, and hung to dry in a mesh bag in her car between swims.