[mousey]

It depends on how warm the tank is as to whether that possible ich will spread to others in short order. Sometimes fish get a spot and it goes nowhere. other times every fish will be covered in days. And of course it may be a small air bubble on the fish at the moment you looked or an abrasion.
However if other fish get spots you are likely dealing with ick. Raise your temperature up to 84 degrees to speed up the process. be aware that all fish and plants may not tolerate the increased heat. There are many good articles about the problem. One thing I recently learned is that some treatments kill the fish faster than the disease-- altho i believe if the disease is in the tank long enough it will kill the fish too. They suffocate.