The phagocytic is a type of algae with a strong ability to form floating colonies, held together by a mucous-like matrix that protects the cells and allows them to rapidly increase in size. With sunlight and nutrients such as nitrogen and phosphate, its cells grow rapidly.
However, cloudy weather can trigger the disintegration of the algae colony into a huge quantity of individual cells, and viral infections cause the cells to burst open. If the wind and the waves are strong, a large amount of foam invades the beach, which is quite common at the Dutch seaside.
Even though this foam might be interesting in a picture, sadly it can be very dangerous for swimmers or surfers.