I'm no expert, but I have 2 possible theories. 1: Heater burn. I saw something similar with a blue gourami that belonged to someone who's tank I maintained. It had a special pattern it liked to swim that happened to include right between the heater and the glass. 2: Light burn. If you had exposed tube lighting, or high output under glass, it is possible that a fish can recieve a contact burn when jumping for food. I haven't heard many cases of it, but I believe it is possible nonetheless.
Hope you get it figured out, leichartdi's look so good in their adult stage, and I wanna see this one get huge.
