Hippothoa flagellum forms adnate runner-like colonies composed of delicate uniserial branches, closely adhering to the substrate. It is a smaller more delicate species than Hippothoa divaricata. Autozooids are slender and club-shaped with a long slender thread-like portion proximally (closest to the colony origin) and a dilated section distally. The thread-like portion (cauda) is one to four times as long as the dilated section.Branches of new autozooids are basically cruciform in the form of a ‘+’, with no more than one bud from each side of the autozooid.
Hippothoa flagellum is widespread in the North Atlantic from Norway to West Africa, and has been widely reported from other regions. This species appears to be practically cosmopolitan in shallow shelf seas. In Britain, it has been recorded from Guernsey, Cornwall, south Devon, Hastings, the Isle of Man, the coast of Antrim and Shetland.
The species is particularly found on shells and other biogenic carbonates. It occurs in subtidal waters ranging from 10-275 m.