if its the only anthias in the tank than my guess its turning into a male. this fish is a schooling fish and requires at least 3 other tank mates. actually i have read odd numbers is best but forget why?
i bought 3 and were all females within 3 months the most dominant one turned into a male and from the looks of your pic its turning into one also they are stunning when finished with their transition
x3...transitioning male Lyretail...if the sides stay a golden color than it's the African(Maldive) variety..I have had both, but not at the same time. Although they can live a long healthy life alone they do best with females present (more than one because their aggression can be spread around) and also their coloration will more vibriant...
here are pics of mine for reference..