This is a tiny Sacoglossid (maximum size is 12 mm 1/2″) and it is often lumped with the many costasiellas that are referred to as “Leaf Sheep” or “Shaun the Sheeps.” These little ones were photographed on their food prey, the invasive algae Avrainvillea amadelpha by snorkeler and marine photographer D. McLeish in approximately 6 metres (20 ft) of water off the west side of Maui. Macro photography at that depth is an incredible challenge for a snorkeler and the fact that McLeish was able to capture the animals in the act of laying their transparent rings of eggs is a true tribute to his photography and snorkeling skills.
The taxonomy of this species is complicated. Cory Pittman, on the Sea Slugs of Hawaii site, discusses this complex species. It wasn’t until early 2023 that this tiny sea slug was given the scientific name of Costasiella fridae.