The valid spelling is y-inversa.

Original description, Limacodes y-inversa: Packard, A. S., 1864. Synopsis of the Bombycidae of the United States. Proceedings of the Entomological Society of Philadelphia. 3: 341.

Moth Photographers Group (http://mothphotographersgroup.msstate.edu/species.php?hodges=4667), BugGuide, and the Pohl et al. 2016 checklist have used an -um spelling, but it appears to be incorrect.