Beneath, and in front of, the green Acropora sp. is a Hydnophora rigida and encrusting the rock to the right of that, and in the right picture, is a Hydnophora exesa.

The H.exesa is one of the surprises I incorporated when I set up the tank. It was encrusting the back of its rock and could not be seen from the front. Now it has crept around to the front on both sides.

Although of the same genus these two animals are not happy together and the H. exesa has stung and killed part of the H. rigida.