What could this be?

Don't do a fresh dip, old simply be a discoloration due to some stess

 

Don't do a fresh dip, could simply be a discoloration due to some stress

 

hard to say, I see similar things like this on many fish though. And then it goes away.

 

Had a convict tang that would have that. Its color also varied with stress like mthomp said. When the lights first came on it looked terrible but would clear up shortly.

 

Just keep an eye on it to see if it gets worse. What all are you feeding?

 

Turbellarian infections (A) are particularly common in tangs, they are free living (facultative) worms related to trematodes. Infection results in dark foci (up to 1mm visible with eye) in the skin best seen on light colored fish (B).

Adult deposits in sediment after 6 days of feeding and produces upto 160 progeny. Cycle much like cryptocaryon) takes about 10 days. Diagnosis by foci of skin, also can be detected with sediment, although not all worms in this phylum are facultative parasites.

Treat with formalin or organophosphate, responds well to freshwater baths.

See "Fish Disease Diagnosis and Treatment" -Noga

 

More specifically, it is probably a member of the Order Nemertodermatida. Since Turbellaria is a wholly paraphyletic group containing Turbellaria, Trematoda, Monogenea and Cestoda, the former of which are obligate parasites. Within Turbellaria is Acoelemorpharia and Nemertodermatida which are both normal and common fauna living in marine sediments.