I noticed this discoloration on the tangs head tonight when the lights were off. When the lights are on, its not really noticeable. It normally sleeps in the rock at night, so I didn't notice it before. I happened to try feeding mysis shrimp tonight and it came out to eat it, which is why I was able to see it. I recently purchased this aquarium already setup and established, so I'm not sure if this was always there or not. Any idea what it might be?