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.
I brought this up in your first thread yesterday and never got an answer.....
I am pretty sure that if something is growing on the top of your rockwool that it is algae and not mold. And they are quite different. Mold is deadly and you should never smoke anything moldy. Algae is entirely different and while it can cause problems, it is not the problem that mold is.