Blue tang have something on the side

[Amiaki]

Please Haley me to identify what's wrong with my tang.

Water
Am-0
Nitrite-0
Nitrate-5
Ca-430
Alk- 8
Mg- 1320
Ph. 8.2

Had my tang for 3-4 months so it's not now to the tank. But couple days ago I noticed something on the side. One black dot about half of the penny size, tang is eating good and swimming around...

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[Amiaki]

Trying to take a better picture but he is hiding when I'm sitting with camera, as soon as I leave he is coming out


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[Amiaki]

Up


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