are you certain that you have mold appearing on your rockwool? Many times the exposed rockwool or nonsoil medium will get some algae growth and some that is white but it is not a problem for the system, especially if there is plenty of light on it and there is plenty of oxygenation of the water in the system.
Can you take a pic and post it here so that we can see it? You really don't want to add fungicides to your grow iff you don't absolutely need them.