Most f/w fungus would not live in s/w nor vice versa. Do not add salt to tank though.
Put the rocks in separate container/bucket filled with salt water is better way to do this. However, unless organic matters are removed, fungus may/can return once rocks are returned to f/w
Like I said, depending on the amt of organic matter on the rock, it may return. I even scrubbed few times until none returned.
Were they labeled as Lace Rock?