After spotting these white swirls, I looked around and found the animal responsible for creating them:
The mystery object above is an egg mass laid by a Shag-Rug Nudibranch (Aeolidia papillosa). There are three nudibranchs (sea slugs) in the photograph (two on the left and one on the right).
With their tentacle-like cerata, Shag-Rug Nudibranchs can look very similar to their sea anemone prey. Here's a different individual photographed in Mendocino County in 2008: