+1 Also check your alk. The easiest thing might be to do a water change (10%/15%) and bring all the parameters closer to the recommended levels. Leathers are strong. I lost the head off my toadstool once. It fell off and looked like a rotting mushroom, but the stalk was still strong. It grew a new top.
did you move the water flow?? or location of the coral? my just simple cause of flow or lighting as well.. when my old one I had would lose color and shrink I realized my power head moved or lighting shifted