Hippo tang coloration is off please help

[Hozat24]

Hello everyone! 2 weeks ago my hippo tang started to change in coloration on his face, he is starting to form black lines on his face (under his eyes) although the fish is eating like crazy and swimming fine! Im so curious to know what is can be! Im just trying to be safe and ask my reef2reef famiy. i have attached multiple photos below. Thank you!

 

[Big G]

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