For anyone to help you, you will need to provide the stats of your water and tank, the template for which is at the top of the tropical fish emergencies forum.
However in my opinion it is likely to be whitespot caused by bad water quality, possibly as a result of not cycling the tank causing ammonia or nitrite poisoning, overcrowding or a mixture of the two.
You need to do lots of big water changes starting with one right now. Also if some fish have whitespot it is unlikely that the others are 'fine' as it comes about as a result of poor water lowering the fishes' immunity to disease. It rarely 'just happens' in a healthy well cycled tank.