Whats going on with this coral?

[lvreefer]

I saw this online and was just curious about what causes this to happen? I am talking about the “arm� that is missing. I recall having this coral in my tank several years ago and the same thing was happening. The arms seem to dissolve over night. Not all at once. Just one here or there, over the coarse of several weeks. Is this a nudi, snail, or some other kind of pest? Or is this a sign of water quality?
http://www.intmarinefish.com/images/fresh.jpg

[graveyardworm]

I could be wrong, but the one you have pictured there appears to have been recently fragged. I have had a soft coral do as you describe and just sorta dissolve leaving behind just a pile of the tiny spicules. I think its caused by an infection inside the vascular system of the coral, once it starts there doesnt seem to be any way to stop it. When mine did it I tried to frag away some branches which were no where near the start if the infection and they dissolved just the same.