Yellowing, spotty leaves on phalaenopsis
 

Hi all! Wondering if anyone can help me identify the problem with this phalaenopsis? The plant gets watered or fertilized once a week, all of the roots look great, is in a bright well ventilated area, and the 14 other plants (a mix of phals, zygos, and dendrobiums) in the room are all fine. TIA for any help!

I can't see the photos very well.

Was that the lowest leaf? I see some raised brown areas on the underside. How long have those been there?

The things that come to mind are:

• Older leaf dropping normally;
• Spider mite damage on underside;
• Some other kind of insect damage I can't see.