Originally Posted by nicetry
It's a viral infection known as lymphocystis. Common to tangs, angels and butterflies, particularly newly imported fish. It will normally clear up on it's own given good water and a good diet. Perform regular water changes. Give the tang a steady offering of greens as well as meaty foods. Soaking foods in supplements like selcon or zoe can also help. In the rare event of a very stubborn case, placing the fish in a quarantine tank and performing hyposalinity can help. It's usually not a cause for concern.
I concur with that.
If you don't mind, I am going to use your picture to help others in the future.
It would be helpful if you could keep us posted on this, in this thread, as you attempt to address the issue.