This slug is typically between 2 and 4 mm and is primarily black in colour with opaque white or transparent tips of rhinophores, cerata, foot, tail, and periocular area.

Costasiella nonatoi rapidly digests the plastids of the algae it feeds on, therefore not carrying out functional kleptoplasty.

[8] Slugs of this species tend to burrow into the thallus of Avrainvillea to feed as well as deposit their egg masses.

Costasiella ocellifera, a closely related and visually similar species also resides on this algae, but tends not to burrow within it.

C. nonatoi may feed on unidentified species of algae related to Bryopsis and Pseudochlorodesmis according to barcode analysis.