This is an unusual crab. The edges of the carapace are expanded outward (like a shield). So although the legs are visible in the picture above, when they're pulled in the legs are completely concealed (when the crab is viewed from above).
Here's a look at the crab's underside. Note how all of the legs fit entirely inside the edges of the carapace (including the first pair with claws, even though one is flexed in the photo).
Also note that the abdomen (triangular piece between the legs) has an interesting pattern, somewhat similar to scutes on a turtle shell. [This species is also known as a Turtle Crab, probably because of the way the shell covers all of the body parts. But I also wondered if that name could have been derived (in part) from the abdominal pattern?]
Umbrella Crabs are highly variable in color. Here's another example (below). [And for a wonderful overview of the many color patterns in this species, check out these photos on iNaturalist.]
The genus, Cryptolithodes, means "hidden like a rock." When the crab is viewed from above with its legs tucked underneath, it can look very much like the surrounding rock! (I hear they're eaten by fish and octopus, so perhaps there's an advantage to being well camouflaged.)
These crabs are usually whitish underneath; pictured below is the same individual as shown above. (Can you find the short, red-and-white striped antennae?)
I haven't observed it myself, but I read that Umbrella Crabs eat coralline algae (!).
One more view — a close-up of the eyes peaking out from the sides of the shelf-like rostrum:
Let's hope that thinking about Umbrella Crabs leads to a need to carry an umbrella!