Powdery Mildew on clones?

I recently cut a dozen or so clones from 2 different mother plants. About a week later I go to look for roots and all I see is powdery mildew starting to form on the stems along with algae. I'm using a DWC oxycloner. I was told to use phed tap water and not RO.. but I feel like the nasty city water could have caused this..

So..
Where did the powdery mildew come from and how can I make sure for it to come back. Did it come from the mother plants?

I have never had visible PM and am very vigilant.. Let me know what you think could be the source and how to combat and prevent its return.

Thanks!!