Its definitely receding into itself, usually a sign of a param issue but if all has been stable maybe something too close to it that it does not like or possible starvation if its not fed at least once a week. Acans are real food pigs & love to eat daily like Blastos. Its not a lighting issue have you kept CAL, MAG & ALK all at a slightly elevated lvl they like their minerals for growing too so REEF mineral lvls is a must can you show your params maybe that might help?