Yellowing/Withering Psychopsis leaf

[Syrinth]

I bought a Psychopsis Mendenhall from the Orchid Show last month and just noticed that today it's smallest leaf is turning yellow/withering.

It is in a south facing window, behind a rather large cymbidium to block out the most direct rays of the sun. The sky has been somewhat rainy and cloudy lately, so light may be a factor.

There are also two East facing windows in the room but it is farther away from them.

Before watering, I check the bottom of the pot to see if it's mostly dry. Most waterings I put a pinch of MSU fertilizer in the water, and I'm currently using tap water. Might start just buying pure water from the grocery store soon though.

The spike has been growing steadily and extremely well
since I brought it home.

Please help me not murder me plant!

Leaf closeup
Plant as a whole