I wouldn't say it's dead. The stalks and polyps are still intact, but the polyps are withdrawn. Something is definitely bothering the coral. How much light are they receiving, and how much flow? Mine always do best in bright light and a fairly heavy flow. Not sure what I'm seeing on the rocks around the xenia, but it favors cyanobacteria. If it is cyano, then there's definitely not enough flow to the xen. Xenia like their water a little "dirty", but too much is not good for anything in the tank. What are your water parameters?