This nudi (risbecia tyroni) is frequently found on pairs in single file, the slug at the back using its head to maintain contact with the one at the front. Tyron’s nudibranch as they are called have an ingenious defense system, just like other nudis they are fond of eating poisonous prey.
Indeed, one needs a keen eye to find them, they are silent and most often lurking on soft corals. We found this in the depths of Puerto Ptincesa, while the viz was far from good we caught sight of it in the yellowish reef almost hiding in the colors. But it was all alone, silently waiting perhaps for its prey.
Have you encountered this nudi?