Looking at your pics and reading how quickly this happened then I would suggest that it is sunburn. Phals in particular can not cope with direct sun on their leaves.

Hmmm, I thought it may be that too. But I actually don't give it direct sunlight? I always place it by the window but I never turn the shades to where it hits the orchid directly. The yellow leaf is also becoming a lil crispy too. D:

I agree with Terry, it appears mechanical like sunburn. The rest of the plant looks good, so I wouldn't worry.

Jane

Took a look at the picture. The main plant looks good. But I'm not sure if the yellowed leaf is sunburn. It is dessicated. That particular leaf is old. The shock of transplant and the lack of established roots probably contributed to the demise of the leaf. The main thing is to keep an eye on the main, green plant. Mist the plant, to create higher humidity so it will help in rooting. Don't keep the moss sopping wet. Let dry between waterings. And never let water sit in the crown of leaves overnight. This can cause crown rot.