Im going to guess its just its spots. It was from my LFS in a 120 Gallon tank for a few weeks. At 4 inches it shouldnt be stressed in my 40 gallon.(Im upgrading to 150+ in a few months after new room done).
Make sure he is eating a diet that is vegatable based. They make a flake food called spirilina that is high in viamin C. Also if he`ll eat dried seaweed or some grape calerpa that would help. He`s a herbivore so make sure he gets plenty of greens.
You can view many of my fish and corals in my photo albums in my profile.