Hi I agree with Bruce you definetly have spider mite invasion. Sometimes the white paper towel trick gives a false positive and vice versa, depending on temperature maturity and extent of population. Paper towel might just show colouration similar to that of dirt/ dust. On thinner leaves you would see a fine stippling on the upper surface but phals leaves are thick and your photos demonstrate how a phal shows mite damage. Treat all your plants. Spider mite does have worldwide resistance to many pesticide products so rotate your treatments and try and obtain a miticide.