Wow.. That looks just like a powder brown tang. I have one, that just went through 7 weeks of hypo. (See my thread in this forum. I just posted yesterday.)
The tang though doesn't have any visible spots to the naked eye. I have to press my face up to the glass and be within 3 to 4 inches of him, then I can see tiny bumps on both sides of his body with little white centers. The bumbps kinda glisten if one looks closely. It went through 7 weeks of less than 1.010SG water with calibrated refrac. I don't believe mine is ich based on the picture you posted. Mine doesn't have ANY visible white spots to the naked eye... Maybe in a few weeks it will I don't know.
Either way, I'm bordering on giving up as well. Keeping a tank free of ich seems to be impossible. Especially if it's immune to available treatments. I tested my water regularly and it was always around 1.006. IF it's immune to that. What if it's immune to copper?
Or well, I guess in my case, what if the tiny, nearly invisible bumps aren't ich and well are untreatable with copper and hypo....
One has to have a degree in marine biology and marine medicine to successfully keep fish it seems. SO, far, I'm in my second year of the salt water hobby. All I can keep successfully are corals, clams, and anemones. I can get sps to grow like crazy. Fish.. well, it seems to be nearly impossible to keep them healthy.