The top photo shows a Flowery Flounder resting on a patch of sandy sea floor. These fish blend in so well they’re almost impossible to spot. The trick is to spot them on the move (second photo). In motion, they’re quite distinctive with their undulating movement, especially if the pectoral fin is raised as in this photo.
The third photo shows the fish just as it landed on the sand to begin its very fast process of blending in.