Free swimming isopod. My guess is anilocra
physodes, probably looking for its next host.
It looks like a Macfarlands, but much lighter without the middle strip. The rhinophores and gills are much darker.
This appears similar to Okenia sp. 1 on page 126 of "Caribbean Sea Slugs"
About 12 mm long, on hydroids (which look like Pennaria disticha)
Slug 8 mm long, from the underside of a low intertidal cobble at El Tomatal, Baja California. The large pits in the sponge (Oscarella sp.) represent a night's feeding activity by the slug, and those are the slug's fecal strands on the surface of the sponge (low center). The 2nd image is of the same individual and was taken by @lemurdillo (Brenna Green). 4th image shows the specimen as found.
Hessam Ghanimi, working under Dr. Angel Valdes at Cal Poly Pomona, last year completed his MS Thesis on Berthella, one chapter of which shows B. stellata from around the world to be a complex of many species, one of which is confined to the Eastern Pacific. Stayed tuned for the name changes.
Sampled from a midwater tow (200 m) in the Gerlache Strait, Antarctica.