These brown seaweeds produce and store sulfuric acid.  When plants are stressed they release acid breaking down their own tissues and the tissues of other seaweeds nearby.  Seaweeds that look like patchy “camouflage material” may be the victims of stressed, acid leaking Desmarestia.  Many species of seaweed in our area are edible (although some are more tasty than others) but Desmarestia is one seaweed to avoid.