Hello, hello~! How are you, guys? :D
Well, after a long search, I have the answer. I wanted to draw it, so that, the reason could be better appreciated. So, the origen is shampoo!. Yes, the shampoo that spreads the mermaids when washing their hair. Besides being smelling of a pleasant fruity smell, they pollute the fish park! :c
Hahaha, I'm kidding. Actually, the sea foam or spume is created by the agitation of seawater when it contains high concentrations of proteins, lignins, etc. You can read more about this here.