Should I take a yellowing leaf off a rootless phalaenopsis orchid???

[Ray]

It is still resorbing water and nutrients from it, so let it decide when to jettison it.

To save that plant, I'd do the following:

1. Immerse the entire plant in warmish water containing 1 tablespoon of KelpMax per gallon, letting it soak for a few hours.
2. Nestle the base into loose sphagnum that has been moistened with that same solution.
3. Invert a clear plastic bag over the plant and pot to trap the humidity, to slow further desiccation.
4. Keep it shady and very warm, watering as needed when the moss is nearing dryness.