This elegance appears to have gotten stung . I doubt a nutrient issue. The swelling concerns me as it can also be a known elegance disease mentioned below.
What nearby corals are within 3-4" of it?
Elegance can be forgiving in many cases. Was there any changes in water flow? You don’t want the flow to be too hard or too soft. “Just right” means the tentacles flow gently, like leaves in a light wind. Too little and elegance corals will not thrive. Too much flow and you risk tearing the delicate polyps.
The proper flow brings nutrients to the polyps, ensuring they remain nourished. And you’ll see the typical “sweeping” movement as they drift in the current.
Elegance corals ARE prone to elegance coral syndrome (ECS). If your coral has this disease, you’ll see swelling around the oral disc and unopened polyps or a white mucus coating.