If the holder is hit by a Water-type damaging move, it consumes the Luminous Moss and its Special Defense is increased by one stage.
It does not activate if the holder already has +6 Special Defense stat stages (unless its Ability is Contrary, in which case it will not activate if the holder has -6 Special Defense stat stages). It does not activate if the holder is protected from or unaffected by the Water-type move (e.g. due to Water Absorb or Wonder Guard).
If multiple Pokémon each holding an Absorb Bulb or Luminous Moss are hit by a Water-type move at once, each hit Pokémon will consume their Absorb Bulb or Luminous Moss before Symbiosis transfers any item (including these items).