Phals can take up a lot of water from a small root system. They can support an amazing number of leaves with a small root system, if properly watered. That means water on the roots, not just humidity around the plant.
The developing leaf is taking water from the oldest leaf. As the new leaf keeps growing, it will do this to the oldest leaves, one by one, unless you start watering the roots.
Take the plant out of the terrarium. Notice the roots are tan, white or silvery. Dip them in water for a few minutes. Notice they turn dark green. The velamen coating of the roots is full of water. Return the plant to the terrarium.
As soon as the roots have turned from dark green back to tan, white or silvery, you can repeat the dunking.
A kelp product like KelpMax can help stimulate more roots.
When the plant is making more roots, you can repot is.
Mold seldom kills orchid roots. It is more likely they were already dead, and fuzzy mold grew on the dead roots. Most dead Phal roots are killed by overwatering. Here is a good thread to read on caring for Phals:
The Phal abuse ends here.