White mold in s/h

[Buds!]

I have had a great run with my phals in s/h, I am using 'take away' containers and PET cups for seedlings.

However I was shocked today to see that 3 of my phals in the plastic PET cups have fluffy white mold growing near the drainage holes and on some of the Hydroton pellets!
I took out the phal, it has a few new roots, but some of the old roots also had small bits white mold on it.

Can this be caused by the cup being too 'tall'? Therefore restricting ventilation to the water reservoir?

How do I treat this problem? Do I flush out with Physan 20?

Thanks in advance.
Buds!

ps. There is plenty of air movement in my growing room. The ceiling fan is on 24/7.