Symun Fossoway is a member of House Fossoway.
Talla Tarly eagerly greets Sam as he arrives with Gilly and her baby, complaining about her arranged marriage to Symun Fossoway. Talla appears unenthusiastic about the arrangement, presumably due to his appearance (she mentions that he has yellow teeth before being cut off by her mother).
In the books
In the A Song of Ice and Fire novels, Talla is not betrothed to anyone in the books, and no character named “Symun Fossoway” has been introduced in the novels yet.