Heat and Salt are the best meds, IMO..
Looks like it could be the start of a fungus, or the rays disc could be 'degenerating'.. Sounds like someone could also be nipping at the claspers or disc.. What other fish do you have in there? No plecos or polypterus, right? No Cichlids, ya?
Also, like he said, double check the ammonia..
Is the ray eating? I have seen discs literally disinigrate from malnutrition..