This is what Beth told me to do. Maybe it will help your fish too.
"That white spot looks very bad. If you can QT, you will be better able to actually see what is going on. It is definitely not ich. If you don't have a hospital tank already set up, then you will need to really babysit a recently set up hospital to make sure that the tank does not have any ammonia or nitrite spikes.
The other thing I was thinking of was brooklynella, which is common to clownfish. Brook usually kills fast, however, wo treatment. You could do both treatments. With brook, you will need to place the fish in a small container with water from the tank [small bucket] that has an airstone in it. Use formalin to do formalin bathes for 45mins, every other day, for 5 treatments. You can easily do both treatments once the fish is in the hospital tank. Antibiotics in the hospital tank, lights off, do a water change just before each dose of Maracyn2 for SWF, double dose for 5 days. Then, remove the fish to also do the formalin bathes.
If the fish is too weak, then just do the antibiotic treatment for now."