These are commonly called Soldier Crabs and there can be thousands of them walking over the sand. I hadn't seen any for some time - I guess crabs must also have seasons or cycles? They are very quick at moving away if they hear/feel steps close by. Sometimes they just head for the nearest puddle of water and quickly disappear. It's really surprising how quickly they can 'sink' under the sand.
Looking south it was very dark. There were a number of Gull-billed Terns swooping down and picking up food from the sand or shallow pools of water.
There was quite a strong wind blowing and shorebirds usually find a sheltered area to rest or feed . Apart from the Terns this was the only bird I saw. From a distance I thought it might be a Red-necked Stint but as I got closer I could see it wasn't quite the right size or shape. It was against the light which made ID harder.
I moved away a little and tried to get the light shining on the bird - but I had only one quick chance before it flew off. I managed one photo with it partly obscured by a small mangrove plant and one more photo as it was stretched out just before it flew. It was a juvenile Sharp-tailed Sandpiper which is not a species I usually see here on the foreshore.