Macelloides is feminine because Uschakov treated it as feminine by using a feminine ending on the adjectival specific name, antarctica. Although Antarctica is a feminine noun as a Latin place name, here it most probably was used as an Latin adjective to which the alternative masculine was antarcticus. Evidence to the contrary is lacking, and antarcticensis was another adjectival possibility for a species named for a place name. Additionally, Uschakov (1955) had earlier used feminine adjectives for his genus Macellicephaloides, thus indicating he regarded -oides endings as feminine (they were either, but are now deemed to be masculine, if other indications of gender are lacking (ICZN Art.188.8.131.52).
Uschakov, P. V. (1957). [On the polychaete worms fauna (Polychaeta) of the Arctic and Antarctic]. K faune mnogoshchetinkovikh chervei (Polychaeta) Arkitki i Antarktiki. Zoologicheskii zhurnal. 36(11): 1659-1672.