Small brown oval disc's

[Bobkap]

I have a collection of Phal's in doors on humidity trays.
I just started to notice brown small oval shaped disk's or shell like growths. They are on the top of the leaves and the underside as well. They are small and I don't think my camera can capture the images. They come right off with your finger and when you run the plant under water they fall right off. Some of the spikes have them all around the spike.
Does anyone have any info is it a bugs shell or a growth?
I looked at them under a magnifying glass and the underside seems hollow like a shell.
No new plants have been added within the last 7 months they are on most of the plants.
Can anyone on the board suggest?