I'm sure this is nothing (ok, I'm not sure), but it worries me since it's so close to the central disc..
On the right side, there's a missing "scale" on one of his arms, right near the central disc.
As for his central disc itself.. well.. this was taken right after he went nuts stealing all the Firefish's food.
I've seen the Firefish nip at his teeth once, a crab do the same, and an amphipod go after his central disc (said amphipod was then picked up and flung into the refugium intake by said Serpent Star).
Is this anything to worry about?