#scum  a film of impurities or vegetation that can form on the surface of a liquid
  supertype:  film  a thin coating or layer; "the table was covered with a film of dust"
  subtype:  slag__scoria__dross__dros  the scum formed by oxidation at the surface of molten metals
     subtype:  basic_slag__basicslag  slag produced in making steel; low in silica but having large amounts of calcium phosphate; useful as fertilizer

No statement uses or specializes scum; click here to add one.