looks like it's starting to "melt" or in other words die. what kind of lighting do you have over your tank? i've had that happen before and was due to insufficient lighting. either that or if it fell off that spot you have it and into another coral, it might have been stung. also make sure you hit it with a powerhead or a turkey baster and knock off all the crap that gets on the tissue. detruis likes to collect in between the extended polyps and "melts"the tissue that connects the colony. this is actually a good practice to perform weekly on all of your corals. you'll be suprised at how much healthier everything will look.