Perhaps you can get a sense that this is an animal with a circular shape? In the picture above, it's mostly buried below the sand.
Sometimes, we'd see a vaguely circular outline followed by a faint trail:
Below, I'll show the entire animal, no longer covered by sand.
So this is a warning — the next photo will give away the answer:
Did you guess sand dollar (Dendraster excentricus)? This was probably difficult, because you couldn't see much of the animal itself. The mystery photos only revealed the outer rim with short spines and slender tube feet. Sand dollars live partially buried in the sand where they gather and consume organic material from the sediment (deposit feeding). In some situations, they capture food particles from the water (suspension feeding).
We saw individuals of various sizes and colors:
The smallest were less than ~1 cm across, indicating they were probably quite young (less than a year old?):
Have you been seeing sand dollars, too? If so, we'd love to hear about it.
P.S. For some microscope views of a small sand dollar, see the post from 12 January 2013. And for information about sand dollar skeletons, review the post from 11 September 2012.