The Mussau Flycatcher is endemic to Mussau Island. A 2014 survey observed males and females in primary and mature secondary forest and found a nest with one chick in a tree fork.
In Hartert’s 1924 original description from Mussau, the adult male has short crown feathers, so the cap is not prominent. The sides of the head and ear-coverts are blackish and nearly matte; the throat and chest feathers are short and loose, with little color contrast between their tips and bases. The adult female has a deep steel-blue crown and a rufous back. Her dark brown wing feathers and tail have rufous edging, which reaches the outer web of the outermost tail feathers. Juveniles resemble females but have a dark brown crown, a yellowish bill base, and a rufous-buff wash across the chest.
During an 11-day survey in October 2014, the species was recorded in primary and mature secondary forest near Nae and Lolieng on Mussau; it appeared more numerous in forest near Nae. Males and females were most often observed in dense understory, and the bird was not recorded in adjoining young secondary growth or old gardens. One simple cup nest was found in the understory near Nae, in a tree fork about 2–2.5 m above ground, with one chick.
An extinction-risk assessment has not been added yet.