Don't see too many tanks with 0 nitrates... Alk is a little low (and ca a little on the higher side). In a softie tank, water changes can help balance them out (depending on which salt you use-some are higher in ca, alk, and/or mag than others). Run some carbon and give them time. Leathers tend to close up every once in a while and 'refresh' themselves. Is it a waxy tissue coming off, or does it actually look like the coral is disintegrating? If waxy, that is fairly common for toads. How much flow is it under? They generally need a little bit to help with the shedding process. If not disintegrating or getting covered in black slime and falling apart, just give it time (not all falling apart is something to be highly alarmed about initially-mine self propagate by developing white lesions and dropping pieces). I'm a big fan of carbon to help clear up chemical warfare, among others nasties in the water.
