Get all of your fish in quarantine NOW. I cant be sure what you have without a picture, but when my tank got an outbreak of velvet, I lost 4 of 12 fish in 3 days. If I hadnt put them in quarantine and started treatment within a day, I would have lost all of them.
If it is velvet, the only treatment that will work is a freshwater dip before placing them into quarantine to give them a fighting chance for a day or two, followed by 1 month of copper treatment and 1 additional month in quarantine without copper.