Pebble crab is a unique creature

I found them on our beach in Aceh, north of Sumatra. They do look like pebbles. As the waves reveal their hideouts in the sand, they run around, and quickly bury themselves back in the sand. I had to lure them out of their hiding with a little scraping of sand. They run away, and show funny behavior.

According to http://www.wildsingapore.com, they have body width 1-2cm. Body smooth somewhat rhomboid, indeed resembling a tiny pebble. (Source)

The head forms a blunt pointed tip with a pair of tiny eyes. In this way, its eyes can peep out while the rest of the crab is buried underground. It has powerful long flat pincers with pointed claws. The crab can bury itself in the sand very rapidly. (Source) I also have noticed they have some different colors.

Pebble crab is Leucosiidae, ie a family of crabs containing three subfamilies and a number of genera incertae sedis. (Source)