It would be helpful if you could post a good image.

Scale crawler stage does appear as white specks, usually at the base of, and along the stipe, then on the underside. Several species of ferns have a structure called hydathodes. The hydathode is the apparatus of guttation that consists of stomata-like pores in the epidermis and the epithem, which has a large chamber with masses of thin-wall parenchyma cells and a sheath layer surrounding its tissues (Kramer, 1945). If your ferns are watered with high mineral or tap water white specks may form in patterns on the fronds.