yellow tang
If you got it two minutes ago the problem is probably just stress.
There probably isnt a problem.
Are the stripes vertical?
If they are, I think that is just the way the muscle fiber in the body is arranged, and the brown may be a result of that.
My yellow tang will be awoken in the morning and it will have a brown area on it near its lateral line for about 10 mins but it will go away. It is very healthy, vibrant yellow.
Try feeding your tang romaine lettuce NOT iceberg, romaine.
The brown stripes are probably just normal.