It could just be that another coral is outcompeting it. I used to have a colt coral that grew to occupy 1/4 of my 125, stretching from it's base in the middle of the tank up to the surface, down to the bottom, and front to back, and side to side. Then, another soft coral (think it might have been a tree, not sure) started to get bigger and bigger, and the colt got smaller and smaller. It could be that one of your other corals is releasing toxins into the water to discourage other corals from flourishing and competing with it for food, and that it is only affecting the one specimen so far.