Rostanga muscula is a species of sea slug, a dorid nudibranch, a marine gastropod mollusc in the family Discodorididae.

[2][3] This dorid nudibranch is bright orange to reddish-orange, and the dorsum is covered with caryophyllidia; it is very similar to other species of Rostanga.

[2] This is a common intertidal species.

It feeds on the red sponge, Microciona coccinea (family Microcionidae).

It is also found to eat the introduced sponge, Ophlitaspongia seriata (=Ophlitaspongia papilla) and the native Holoplocamium neozelanicum[4]