So he is terrified of you? You have mentioned a few times now that it swims away or hides from you.
Think of it this way, a person caught him, a person put him in a bag and shipped him to where ever, a person opened the box, put him in another tank where all day long more people looked, tapped on the glass, etc. Until you, another person bought him, put him in your tank. Pretty traumatic for a fish.
If there is no slime oozing off his body, let him be. Turn the lights down sit far away from the tank and let him watch you and learn that your not bad. When feeding, put it in and move far away from the tank.
It may be a disease, but it is more likely that you will kill it trying to treat it for a disease it doesn't have. If it makes it, great, if not try the things I mentioned from the beginning, and make the LFS
show you the fish is eating before you buy it, until they can be trusted.