[chandigz]

From the picture it doesn't look like an infection but just straight recesion which makes me think water conditions. I'd check all water paramaters first (Ph, Kh, cal, mag, ammonia, nitrite/nitrate, phosphate). Make sure they are within range and post if possible. It is hard to try to diagnose a problem without knowing them. Check for brown jelly or white goo around the areas of recesion. This would indicate infection. You could try an iodine disinfecting dip. Is anything picking on it or is there something close that could be stinging it. Fragging a wall hammer is difficult and would overly stress an already weakened coral.