I'm afraid the formalin is only effective with black ich. It is not as contagious as the white, so there is a possibility that your other fish won't get it so I would effectively treat the infected fish following the directions posted in the FAQ and then wait and see if you notice any on the others. If you see any of the others with this, then you will need to treat all of them. I don't recommend it right off, because it is a hassle treatment [having to remove fish to bath 3-5 times and it is a rather potent drug that should not be used unless necessary].
With white ich, you will need to treat all the fish. What type of setup do you have, do you have a refractometer and a QT?