Looks like popeye. Usually a bacterial infection that was acquired from an injury or poor water quality. Possible outcomes if you just do nothing. Sometimes it goes away, sometimes the fish goes blind on the affected side or maybe even death will occur. If you want to take a proactive measure, bacterial infections are usually treated with antibiotics. My preferred "reef safe", use at your own risk, is seachem metroplex and/or kanaplex with seachem focus. Unfortunately, it takes awhile for antibiotic treatments to be effective in fish. Antibiotics have been known to decrease fish appetite as well, so you may want to add a supplement to entice the fish to eat. I recommend selcon or garlic. Garlic's medicinal properties are a dubious proposition at best, but it does entice fish to eat.