I had great success recovering "melting" zoas by dosing ascorbic acid (vitamin C). It has been shown to reduce oxidative stress in corals and also to stimulate collagen production. There is a long thread on in the Zoanthid section on RC with this discussion. Is it just this colony showing this distress? I would worry about those white, lumpy growths at the base of the polyp, suggests pox or fungus as mentioned before.
Cheers,
Josh