These "sea creatures" are about 2 - 3 inches / 5 - 8 cm long and wiggle. They have the same patterns/colors as the crabs who crawl over the rocks. I tried Google and several apps but am unable to identify them. So if anybody knows....
We saw them yesterday at the little beach at Bandar Khairan. I have seen them before, but getting a clear photo of them is tricky. But now I have one, I will ask in the hike whats app group too...