He is stressed. Feed him lots of food laced with drops of freshly squeezed garlic. They are hardy fishies and yours still has some fat. He loves Nori as well, drip 4 drops of garlic on it and pre-wet it with RO..
Garlic drops: Peel a head of Garlic, Liquidize in a glass with a quarter cup of RO. Sieve in a tea strainer and keep 1/4 of the juice in the fridge and the rest in the freezer as it does go off.. You will smell it when it does. LOL!
They become dark and mottled when they are scared, sleepy, upset. And bright yellow when happy.. Mine is always hungry, and quite a friendly fish.
Good luck.. Tagging along to see what happens.
Oh ja and if it is stressed it also swims tail down.