Hi Dirk -- From the square shape & the position of the eyes it might be a grapsid. A lot of grapsids are algae pickers, predators on smaller or similiar-sized crustaceans & other inverts, and scavengers. Around here - California - they're very common in the intertidal, constantly picking at edible bits on the rocks.
If you keep the crab completely submerged when you photograph it the pictures will show more detail.