Where is it growing from? Could be over fed, this often happens.
Please do not EVER throw a live fish away!!! Only if it is necessary should a fish be euthanised and in that case they need to be done so in a humane manner, one which kills them quickly and painlessly.
It is hard to know what the problem is without more info. What are your ammonia, nitrite and nitrate readings? What and how mucha re you feeding? What is the size of the tank and what other fish are in it? Also is the fish showing any other symptoms of disease or abnormal behavior? A pic would be helpful if possible.