Help- trying to rescue rootless phal

[ccrow]

I made an impulse purchase of a little noid phal at the grocery store... I know, I should have looked at the roots but it was so cute! Long story short, it had no live roots. I cut off all the dead ones and part of the stem(also dead), and removed the lower two leaves to try to reduce moisture loss. I have it in a wide glass vase with gravel and water; it's in a plastic container inside that, and I have cling wrap over the opening. But if I keep it covered to keep the humidity up, it gets mold or fungus growing on the stem... what am I doing wrong? Or what am I not doing that I should be?