The body of this delicate species may reach 38 mm, white or pink in colour, with three opaque lines down the back. On each side of the dorsum is a row of up to eight dichotomously divided yellow cerata; they each contain a grey or brown digestive gland branch. The first pair of cerata lies anterior to the smooth rhinophores. The anal papilla is lateral, behind the second ceras on the right side. The genital openings lie close to the base of the right rhinophore. The head bears a pair of conspicuous, curved oral tentacles.
H. formosa may be locally very common feeding on hydroids (Abietinaria abietina, Hydrallmania falcata ,Tubularia indivisa and T. larynx . Specimens found recently in the Sound of Mull were feeding upon Nemertesia ramosa , and lacked the pink colour of animals taken from Tubularia , to 60 m.
There are records from the north, south, east and west of the British Isles, but elsewhere it is known only from Norway and the approaches to the Baltic Sea (Distr. H. formosa).