sick duncan

I recently began noticing an issue with my Duncan coral. It has been a little more closed up than normal but not much. on the side skeleton area of the duncan there is a bit of tissue recession. There is a open patch on the upper section that is surrounded by a yellow 'rim'. Where the skeleton...

Newbie here my Duncan has been closed up for about a day and today I saw this pimple on it. Is it a new head or an infection? I saw it stretching yesterday in that direction.