Hmmm, never seen marching ants in Lightroom but perhaps close to your scenario is Highlight Clipping on the “J” key. This displays blue and red masks over black and white clipping, respectively. Reducing the Highlights adjustment will reduce the red mask until the white clipping has been avoided. Increasing the Blacks adjustment will do the same for the blue mask and black clipping. Or you can just click'n'drag the Histogram area.
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