How long have you had the wrasse?
This doesn't look like an injury, or if it was, a secondary infection has set in.
Sometimes fish can battle off a bacterial infection if it isn't too deeply seated in the tissues. However, when I see a wrasse with an "injury" like this, Uronema comes to mind. You most often see it in chromis and wrasses, usually within a month or so of acquiring them, Here is a link to an article I wrote about it.
Red Band Syndrome in Marine Fishes: Caused by the Protozoan; Uronema sp. Jay Hemdal Introduction: This moderately common disease of certain species of marine fish is so often misdiagnosed that most aquarists apply incorrect treatment methods...
www.reef2reef.com
Jay Hemdal