My citron seems to go through periods when he looks quite poor, despite eating, and then he begins to look like himself again. My current citron has been in my care for over a year now, and he has had probably three times of looking like he is knocking on death's doorstep. Part of the problem is that he only likes to eat frozen enriched brine -- he spits out everything else. Sometimes I mix a bit of anti-parasitic food in w/ the frozen that I feed, but the citron doesn't eat it. If you are in the US, most dedicated LFS sell Prazipro. I know That Fish Place has it.
Also, from Matt Wittenrich's Breeder's Guide, Valenciennea, Elacatinus, Gobiosoma, and Gobiodon spp. males often display sunken-in stomachs and appear emaciated, and have a heavier cranial profile (that is pretty much a word-for-word quote.)