Based on the picture I can honestly say I have no idea what the white stuff is. If it just appeared and he did not have it before, I would guess maybe bacterial.
If you have a QT
tank I would take him out and put him in there....one it will keep him from getting picked on, and two to prevent spreading if it is something.
Most likely if he is getting picked on real bad it is something relating to the stress.
For the record.... Alot of fish will go days without eating. In fact most people do not even feed them everyday. I would not worry if a fish does not eat for one day. If it last a few feedings try something else to entice them to eat, soak food in garlic helps alot.
Good luck maybe somone else will know what exactly is wrong with the damsel