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.
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I had a clip of seaweed, I'll replace the piece in a minute. It didn't look like it was eating it. He was mostly eating algae on the rocks. I'll pick up the food tomorrow.