It unfortunately sounds like it might be marine velvet. This is not good, as treatment is usually copper, which is a disaster in a reef, so the fish needs quarantine to be treated, which will stress him even more. Trying to get it to eat some healthy food and trying to reduce stress on him is all you can do IMO
. It might be a small bacterial infection on some damaged skin and might just go away, we can hope for this.
I won't go tang police, but even a 150 is a bit small for a Regal, or most any tang IMO
. When I tried to keep tangs years ago, they always had ICH or some other bad thing. When I built my 300, the tang problems disappeared. They must have room to swim, hidey holes to sleep in and plenty of good food. Otherwise their already thin slime coat breaks down and infections and parasites can attack them. This isn't opinion, it is a fact of biology. So even thou they are a very popular fish in the industry, they can be a problem if those rules aren't observed. There are many varieties of tangs and I am sure others on this forum will add those they think would be okay in a 150. My regal likes his hidey holes and eats an enormous amount of greens. He is also one of the most active swimmers I have.
There may be other treatments you can apply, but they will only be temporary if the overall health of the fish isn't considered, hope that helps some. I sure had my tang problems over the years.