A hydroid colony is a group of polyps like sea anemones. They form a tight cluster and appear to be a single organism. Within the colony are different types of polyps with different functions. Some specialize in feeding, others provide structural support, and others protect the colony. There are no polyps that control locomotion; the colony cannot control its travels. The triangular sail catches the wind and the By-the-wind Sailor is blown across the ocean even in a gentle breeze.
All that being said, a couple of people have posted blogs stating that there was a study showing that the Sailor may be a complicated individual rather than a colony, but they did not include any citations. If anyone does have a citation to the research paper, please send me a note.