That behavior is usually caused by irritation from a parasite. If it's a new addition, check very closely for skin flukes. They're nearly transparent, but can be seen at an angle, with an oval shape attached to the body of the fish at one end, like loose flaps of skin. Other tale-tell signs are whitish scarring on the body and head. But if could be any number of irritants/infections.
If you do find that flukes are the problem, a brief freshwater dip will kill them off. My gramma had what I thought were two - three flukes. When I dipped him, about 10 fell off.