The above pic is the effects of HLLE
and will generally be caused from an improper diet. Tangs are herbavors and require algae based foods regularly. Meaty foods will do little for them. I would suggest you start leaving some nori in a clip for the tang to graze on and soak the foods you use (once or twice a week) in a good marine vitamin like Zoe or Selcon to provide added vitamins. The HLLE
will subside with a little TLC but the scars may only diminish.
If the black dots are indeed turbellarians, the copper and FW
dip should have been somewhat effective unless they where not done long enough or improperly. If you put the tang back after 6 weeks of treatment and there has been a re-occurance then either the treatment wasn't long enough or not enough time passed for them to die off in the main tank. Unlike C. irritans
and the like, turbellarians can actually take up to 2 months to be eliminated from the display tank in some cases.
If you need to re-treat the Tang make sure all
the fish are removed and placed in a QT to prevent the worm from keeping a strong hold. I would suggest >>hyposalinity
<< instead of the copper though. It is far less stressful to the fish and 100% effective against the pest when done properly for the correct duration. You will also need to leave the tank fallow longer..
There can be no fish in the main display if you are to be rid of it.