Yeah I have seen that before too. Mine does it when the lights are out. It's that same look but this is in a small square on the tail, and it's there 24/7, not just when the lights come on. It's there all day and night whether he is moving or still. I have no idea what that is... Very weird, and very new. It's only been like that for maybe... 3 days now? But the mandarin seems fine, and is hunting for pods and all... so... I DK? If he stops being active, then I will be concerned, Or if it stays for a long time.
Could it be possible that he was stung? or somehow injured? like by a fallling rock or something? I dunno.. That is my best guess.