Sea apples are short, round sea cucumbers. The frilly stuff is the tongue -- err, feeding apparatus, which they extrude to gather food and then periodically swallow so as to clean off the edibles. I usually think of sea apples as not-particularly-exciting wallpaper on the bottom. They seem to have more interesting structure and color, when seen from close up.
A biologist tells me that this is an isopod, of the genus Idotea. You can see it crawling down the base of the sea apple above.