Each individual, a zooid, extends a crown of ciliated tentacles called a lophophore to filter algae and bacteria from the water. Zooids bud from one another, so a colony is a clone descended from a single settled larva, and they sit in rosette patterns visible on the surface of the mass. Most of the colony's volume is a firm secreted gel, largely water.
Colonies are annual in temperate water. They grow through summer, and before winter they produce statoblasts, resistant disc-shaped dormant bodies that survive freezing and drying and can be carried between water bodies on birds or boats. The parent colony then disintegrates.
The masses are frequently reported to local authorities as sewage fungus, invasive eggs or pollution. They are none of those, and their presence generally indicates reasonably clean, warm, slow water. What is less clear is how much of the species' recent spread in Europe and Asia reflects genuine introduction versus improved recognition of an animal that was previously mistaken for something else.