Well, I have a pair of Solomon percula that have it on and off over the last weeks. I'm trying API General Cure now.
If it is only this one time and otherwise there are no signs of infection I would just observe it. It may have eaten something bad.
If it persists treatment with Praziquantel and/or Metronidazole may be required. There are medicated foods, but my fish don't like them.