If you ever see what looks like a dead leaf in a coral head, it is most likely a Leaf Scorpionfish. They grow up to about four inches (10 cm) long and wave gently in the water, looking very much like a leaf.
Wikipedia tells me that they are ambush predators. They wait until a tiny shrimp or fish gets close to them, they sidle a little closer and then snap the prey into their open mouth. Yum!