Pleurothallis has white substance in potting media

[etcherkate]

My poor little Pleurothallis gargantua seedling has suddenly grown a white substance on its potting media.

I wonder if the source of the problem is its neighbor, a Phragmipedium Don Wimber. Maybe two months ago, I noticed that the lowest leaf on the oldest growth of the phrag turned yellow and then brown. The leaf didn't become papery and easy to remove as I've observed in my phals; it is soft and firmly attached.

I isolated the phrag from the rest of the plants to see if the other leaves were affected, and when the brown didn't seem to spread, I reunited it with the others and hoped for the best. (The brown at the tips of the leaves is old, and watering the plant with bottled water instead of tap seems to have prevented the problem in subsequent growth.)

Could it be that the two plants are suffering from the same ailment, but showing different symptoms? Is there a way to effectively treat the problem without killing either plant?

Thanks in advance for your advice!