A blue-gray, milk-glass appearance on the buccal mucosa that disappears when the mucosa is stretched is MOST consistent with:

Leukoedema is a benign, very common condition appearing as a filmy, grayish-white, or opalescent discoloration of the buccal mucosa bilaterally. It disappears when the mucosa is stretched — this is the pathognomonic clinical feature. It is not pre-malignant and requires no treatment. It is more common and pronounced in darker-skinned individuals. Histologically it shows intracellular edema of the spinous layer.