This species was described by Alfred Philpott in 1927 using specimens collected at Mount Arthur tableland at 1400m in December.

[2][3] George Hudson discussed the species in 1928 in his book The Butterflies and Moths of New Zealand.

[3]This species is very similar in appearance to M. fusca but differs by being slightly larger in size and narrower of wing.

[6] M. fallax can be distinguished from moths with a similar appearance as it has small white markings on its forewings.

[6] Along with its type locality, this species has been collected at Ruapehu & Mount Taranaki as well as from Nelson to Lewis Pass.

[6] It has been speculated that the larval host plants may be lichen but larvae purported to be M. fallax have been collected from periphyton.