Tangs are algae eaters. They need it every day. Go to your local oriental market and purchase sushi algae (nori or laver). Also go to you LFS and pick up algae clips. Cut the nori into quarters. Fold that quarter two times, cutting the fold each time. Clip this into the algae clip and now cut ribbons about 1/8 wide.
EDIT TO ADD:
Here is the detail with pictures on how I prepare oriental market nori. I use an old cutting board and have put down painter's tape to center the nori. I also mark on the tape the centers, both top and bottom, and right and left. I use these marks to put the straight edge on to cut the nori. I'll then take some nori....don't count it, but maybe 8 to 10 sheets, and put it on the cutting board. Using the straight edge and a single edge razor, I'll cut the strips into four quarters.
I cut the entire package of nori and package this up in a couple sandwich baggies....for later daily use.
When I'm ready to feed, I'll take these small pieces and first fold in half and then cut down the fold line:
I'll fold in half again, and cut down this fold line
This gets placed into the algae clip and now cut into ribbons...about 1/8 inch wide:
When you're done, you now have bite-size pieces of nori for your tangs.
I have two tangs and feed two of these quarter sheets every day. Both these sheets go into one clip.
@jaijai What you are seeing might be a nutritional deficiency or even the beginnings of HLLE. Either way, a change in diet (as discussed above) is definitely required! If you can, post some closeup pics of the fish and the affected areas. So we can help you determine whether or not a disease issue might also be in play here.
Look up HLLE. It is hard to tell by your photo but it looks like there are multiple discolorations besides on his spine. Blood worms (from what I've heard) is your least nutritious option. You are going to want to stick to Mysis shrimp or brine shrimp and definitely nori.