Hard to tell from the photo. Could be spider mites. They cause a variety of visual damages to leaf surfaces, especially the underside. Gray coloration, pitting, small dark leaf spots, redish coloration at infected areas, and in severe cases, fine white webbing is visible. I have found that having a magnifier of some sort is a must for orchid growers. I have a small mag glass at 3x and 5x magnification. With a bright light, i can easily identify spider mites if they are present. Whenever i find a suspect leaf, i give it an inspection. Without the magnification, you will not be able to see the mites. Actually, thats not totally true. Once you see them under magnification, you can then train your eye to detect them on leaf surfaces. But they then only look like reddish-brown dust.
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