your linckia is definitely in trouble. He is not done yet, but certainly on its way..
One of three things can happen at this point.
1. The deterioration will continue and will spread to the rest of his body and he will start to melt away. This will happen rather rapidly if he is in fact dying.
2. His affect leg will fall of but it is possible he can recover from this. I've seen this with my starfish. If they are healthy they can recover from the loss of one or two legs.
3. The leg will fall off and regrow a whole new starfish. This is very unlikly but totally possible. I've witnessed this in my tank once.
Here's a baby Blue Linckia growing from a leg that fell off of the parent starfish.
and the same baby linckia a few months later..
You will truly know if your starfish is done if his main body (mouth area) starts to deteriorate. It'll be evident once you start to see the flesh melting and turning white. At that point it would be safe to say that your starfish is truly done
