The white areas look like either bacterial, or fungal infections to me.
A general cure medication should help.
What is the average temp of the tank?
Most bacteria tend to become most virulent at temps above 82'F.
JDs come from areas in Mexico where the average water temps are in the mid 70sF, and drop a bit during the rainy season, if your water temp is in the 80s, you may want to slowly drop it to mid to high 70s.
The video below is of JD natural habitat, snorkeling there is comfortable, but it is by no means overly warm.
Eden2