MECS - please bear with me for just a moment & we'll get to your answer. I'd like to make a point. If I simply show up and announce that they are a normal part of the plant's anatomy and say nothing more, there is no way to determine if I know what I'm talking about or not, unless you trust me; and it would certainly appear as though I was disagreeing with the probability of it being any of the above for the sake of disagreement. If, however, I say they are normal inclusions called cystoliths, occurring in cells called lithocysts and a normal part of the plant's anatomy, it puts the reply in a different light, which is always how I try to reply to anyone if I don't happen to agree. If these structures are examined under a microscope, you'll see a little stalk or stem-like structure with a cluster of (CaCO3/calcium carbonate) crystals that look a little like a head of cauliflower on top of the stem. Al

I'm certain it's not a rodent. It's on a high shelf in my bathroom- nothing like that could scale tile walls... such a mystery. Also noticed edges of an orchid and another succulent seem to be chewed.

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.

Thank you tj, The greenhouse is very simple, a temporary structure of PVC and greenhouse plastic, with two doors on each end. To ventilate, I just open the doors enough to maintain the temp, so it is not protected from insects or dust. Does this make you think it is pollen? I thought it would be easy to test this hypothesis, but it is currently and has been raining steadily for 24 hours here, so I cannot check plants in the yard to see if they are also covered in these specks.