Many thanks to Brenna for introducing this slug to me through her photos and then in person, because lordy, I would never have noticed if she hadn't pointed it out. I don't think I saw any that were over 1 cm, and they blend in perfectly with the octocoral that they eat. To find them, you basically turn over a rock and either look for the pair of white spots (which aren't always there), or little tan patches of goo that are... slightly gooier than the surrounding goo.