My Achilles tang has a thin white line on the base of his tail by his scalpel. But it doesn't look like it's hanging off him and on his red dot you can see it continue but it looks like it's under the skin. Any ideas
I agree with Bang Guy, but the pic is not very clear. You will want to keep an eye out since it may be the beginnings of Head and Lateral Line Erosion--a disease for which tangs are prone.
Also, it is definitely not a worm. Perhaps if you can get a much better picture posted....
I would keep an eye out for possible development of HLLE, otherwise, if the fish stays healthy, then its just a scar. That area can be vulnerable however, so it will be something you will want to be looking out for.