The headword is attested both with accent on the ultima, as here, and on the penult, but in either case its sense is uncertain: see Cunliffe, p. 273 and LSJ s.v. mo/rfnos, -o(. A likely source is the paroxytone (penult accented) form at Homer, Iliad 24.316 (web address 1), where it is masculine accusative singular (rather than neuter nominative/accusative), and serves as an epithet of the eagle (Richardson, p. 305); for this see also Hesiod, Shield of Heracles 134. Aristotle, History of Animals 618b26 uses the oxytone (ultima accented) form of the headword as a noun, morfno/s; cf. mu 1269. Possibly the change in accentuation from ultimate to penult marks the transfer of the word's usage from adjective to substantive (Chantraine s.v. mo/rfnos).
[1] The headword has the same gloss at ps.-Zonaras 1371.23, along with the additional one me/lan (black, swart).