If you added other livestock recently, they may have brought in something that the flame caught. Otherwise, one of the tangs or the damsel probably is bullying the flame.
Did you run a test to make sure you don't have any ammonia present? It probably would show with the tangs also, but it is worth a check.
As the others have said, the 75 is much too small for either of those tangs, let alone both unless they are only 2-3 inches. They are both large, active swimmers that need a lot of room and your tank is 48" long, right? When they get much past 4", they tend to feel cramped and generally will become quite aggressive to tankmates. You need to find another home for them regardless of whether they are bullying your flame.