Please help

I have these 2 clowns and one appears to look like he has a pimple or got it in the mouth for being a smartass LOL. It's a 20 gallon all checks are good. 1.023 salt, ammonia nitrates and nitrites are 0, ph is 8 temp is 77. Tank has been running for a few months. Cycled with love rock and sand. Stock is the 2 clowns and 1 cleaner shrimp. Anyone have any idea what this could be or how to fix his face. Any info would be greatly appreciated. Thanks