Could be, could be a worm of some kind, could be a fish has changed it's diet as it matures, could be shifts in microbiomes, could be increased competion for nutrients as other corals are growing if the amount fed fish hasn't changed. If you're not doing water changes I would suggest doing starting. Stop carbon dosing if you are, the labile DOC can cause pathogenic shifts in microbiomes. Check pH, alklinity, calcium and magnesium if you haven't already.