Looks like ich but I’m no biologist. Not sure how much research you’ve done but blue tangs can be notorious ich factories. Something to do with a thin slime coat. Ich can be extremely stubborn as well and last in your tank for months or longer without fish and possibly never go away if the tank still holds fish. All fish will need quarantine, not just the tang because you run the risk of just reinfecting him. Just because the other fish are not showing ich it can still be present in the gills. Tricky stuff, It can even be transferred to another tank just through the air. I’ve heard qt tank should be in another room. Some say it can’t be stopped, only controlled. Brstv on YouTube had some good videos that helped me understand the problem.