Necrosis is the flesh rotting/decaying and will usually smell quite foul. It's very unmistakable.
Necrosis is definately bad but removing what you did was not unto itself bad nor should it hurt the coral in any real way. Personally I find that parts of leathers can look "off" but not decaying and will recover quite easily once the problem causing it is eliminated. If the flesh is actually decaying, it would need to be cut away.
Was the area you removed hard or contain little spikes at all?