Elysia rufescens is olive green with large white spots, often in a reticulated pattern, and grows to about 6 centimetres (2.4 in).

Its range includes the coastlines of South Africa, Réunion, Thailand, Myanmar, the Philippines, Japan, Guam, Samoa, Tahiti, Hawaii and Australia.

[3] Elysia rufescens feeds on green filamentous algae such as Bryopsis pennata, which it rasps with each of a series of rachidian teeth.

[3] E. rufescens grazes on Bryopsis sp., an alga that defends itself from predators by using peptide toxins with fatty acids, called kahalalides.

[4] A bacterial obligate symbiont produces many defensive molecules, including kahalalides, in order to protect the alga.