If I’m Clean, Why Do I Have to Dip?By Rabbi Jack Abramowitz
If upon re-examination, the mark’s appearance hasn’t changed (despite being scrubbed), then it is unclean even if it hasn’t grown. The garment must be burned in this case as well. But if the mark faded from the scrubbing, the affected area is cut out. If the mark comes back, the garment must be burned.
If the mark came off from the scrubbing, the garment could be immersed in a mikvah and used. (Why did a garment – or, for that matter a person – declared clean need to immerse in a mikvah? Even though they were free of tzaraas, the fact that they needed to go through the quarantine process contained as aspect of ritual impurity. See Rashi on 13:6.)