I have a couple in my tank. Got them from Upscales. They're amazing little filter feeders. Don't see them out in the open a whole lot, but can usually find them in a spot with plenty of flow. Very cool coloring.
Here's a little snippet on the web about them:
The most interesting motile invertebrates in the tank are two porcelain crabs (Petrolisthes galathinus). They also seem to have preferred areas in the tank where they are reliably found. These locations seem to be determined by the currents in the tank. These crabs are so fascinating to watch because they have specialized “plankton nets” that they employ to sweep the water for floating food particles. In the wild they seem to be colonial, living in aggregations underneath rocks and coral heads. They can move incredibly swiftly to elude predators. However, if they are cornered, their front claws are very sensitive, and designed to break off easily. Once it falls off, the claw will continue to jerk around and snap open and closed (akin to a gecko dropping it’s tail), presumably to distract the predator from further trying to pursue the crab. Porcelain crabs will quickly regrow these lost claws. We highly recommend these Caribbean porcelain crabs to anyone with a reef tank, especially nano tanks. Beware that many of their Pacific counterparts are commensal with carpet anemones, and thus don’t usually fare well in captivity if they aren’t provided with a suitable host.