That tang looks to be a large, mature adult specimen. An achilles tang of that size is potentially 10+ yrs old. These fish are typically more challenging to adapt to captive conditions than smaller juveniles or young adults.
It also looks like you have the tang in a bare bottom QT tank. IME the problem with a BB QT tanks is that they require almost daily siphoning to remove waste and uneaten food in order to avoid degrading water quality. The daily maintenance can keep a fish from getting comfortable and feeling safe, which can keep it from eating. The flip side is that if you back off on messing with the tank, water quality can degrade quickly. What do you have for filtration? A BB tank will need some form of extensive external biological filtration to manage ammonia production.
What do you have for hiding places? Ideally you should have several places where the tang can fully retreat and feel secure. I like to use large diameter PVC or ceramic flower pots for this.
Also, the addition of medications can suppress appetite in many fish. My personal philosophy is to get all fish in QT eating before applying medication, and only medicating if there is an actual need (e.g. signs of ich).
I have had tangs (recently a naso) that only ate nori for the first 2 months. Frozen blood worms was the thing that got it to start eating frozen foods. People have also had great success getting finicky fish to eat with live blackworms.
Hoping for a positive result for you and the achilles!
