That's because it is not eating. Give it time to adjust. I wouldn't even worry if it has been 2 weeks of not eating. Keep the tank lights above the tank off.
Once you reach the 2 week mark and it's still not eating, add epsom salt to the tank (1 tablespoon per 5 gallons for 3 days). Then proceed to saturate floating pellets or freeze dried food with an epsom salt solution (3/4 teaspoon epsom salt and 1/2cup of distilled water).
That I have not decided on yet, wanted him to get settled in and to spend some time with him to see if anything "naturally fits him". Like how Caspers name fits him.