I had a run in with that fungus that wiped out an entire colony. Jennie, what you want to look for is (for lack of a better description) a cheese-like whitish area. It generally is barely noticeable when in the early stages. The first sign is some of the polyps stay closed and begin to melt away. Once a few polyps go it seems to take over pretty quickly. Check around the base of the polyps to see if you have any white filmy looking stuff. If you have any Lugol's or Seachem Coral Disinfectant you can try that, but from what I've been able to find researching this and reading through Borneman's book, it' called Beggiatoa sp., and unfortunately Bornemas says there is no cure for it, although I would think if caught early enough, you could dip the coral and frag off the healthy polyps to try and salvage the coral.
Now, there's also those pesky zoo eating nudi's. Check for those as well.