:O Where did you get him originally? In the wild?
Stars don't react to "new systems" or those with high
nitrates etc. My serpent stars were the "tell all" of my water quality. When they get sluggish, or show lesions of any type, it typically means my parameters are off.
If you didn't get this star in the wild, you really shouldn't release it in the wild. Better to keep it and work with it, or euthanize it properly. If you got it from the wild, you really should
QT it and learn about it before putting it into your system. What if it were infected with bacterium or a parasite, causing the lesion? You may now have introduced that to your tank.
Ah well... best of luck though.
Dave
