why does blue green algae are hydrocolloids????what is meant by hydrocolloids

Blue green algae are commonly known as seaweeds. They are the source of hydrocolloids: agar, carrageenan and alginate and are used for the production of hydrocolloids. 
Hydrocolloid are non-crystalline substance with large molecules. They dissolve in water to give viscous solution. Extracts of seaweeds contain alginate, which was found to have gelling properties.