The Yasabi (野錆, lit. Wild Rust) is a rust like mushi. They attach themselves to the body of dead animals to consume them. They emit a sound while they slowly decompose their host. Whenever other Yasabi hear this sound, they travel to the sound's place of origin in search of food.
But if no food is found they attach themselves to anything including living beings. The attachment of the mushi makes the host have stiffness in the joints and sometimes cripple them.
In this story, Shige's voice contains overtones that the Yasabi mistake for their own call. They are attracted to Shige's village thinking there's food near her, but not finding any leads, they resort to attack the villagers.
Yasabi dislike and are repelled by Shioke (tidal lifeforce / 潮気) or saltwater. Breezes from the sea will make them disperse.