It all depends on how severely it was stung. If you took it off timeously, it should recover fast. (obviously dependant on water params and conditions) Coral will first shed tissue before polyps start bailing out. If the tissue is shed(hence white skeleton showing) but polyps are still present, recovery will be fast. If no polyps vivible, frag it, as alage will grow on the exposed skeleton.
Also see if there are any mesenterial filaments from the monti still attached to the acro, if so, frag off the part of the acro where they are attached. They will carry on stinging tyhe acro for some time after the corals came into contact.