Little leaf on Phal has started to turn black

[rbarata]

And what's your temp? How cold is it?

I suspect the cause might be the watering from the top together with too low temperature.
Phals should not be watered from the top because the water tends to be trapped in the plant crown. This, together with lower temps causes crown rot.
I also suspect watering twice/week is too much.

I would take the sphagum out of the equation, keep it in bark only, water it once/week, never more, and always in a way to avoid water on the crown and leaves.

Anyway, let's see what more experienced growers say.