I said to myself, "Huh...do Two-spotted Keyhole Limpets (Fissurellidea bimaculata, formerly Megatebennus bimaculatus) eat sponges?" It certainly looks like the limpet (relatively small, dark brown shell surrounded by a bright yellow mantle) had been eating the green sponge (Halichondria sp.).
So I read the species account in Intertidal Invertebrates of California which said this:
"This species often occurs on compound ascidians where the color pattern provides good camouflage. In the laboratory, Megatebennus has been observed feeding on compound ascidians, and sponge spicules have been found in the gut of specimens collected in the field."
And then I laughed, because the next photo I found was this one:
The Two-spotted Keyhole Limpet above is in the center — with a dark brownish shell (barely visible) surrounded by a pale yellow mantle. It's flanked by several compound ascidians, and you can see how well camouflaged it is.
The description in the species account by Don Abbott and Gene Haderlie was spot-on!