Roots about to hit gross humidity tray water
 

The air roots of my phalenopsis are about 1/2" from hitting the water in the humidity trays. I don't keep those super clean. It's mostly the crusties from hard water drying, but probably some leftover fertilizer, and I'm sure bacteria. I've never put anything in them (e.g., bleach) because I don't want the evaporation to harm my plants. But I never get a chance to clean them because of where they're situated and I never let them get dry. I'm guessing this is bad for the plant? Just confirming...

If they don't want to go there, they won't.

You know these plants grow in the jungle, right? :evil::evil: Algae and bacteria are not a problem for the roots.

I have a Dracula orchid whose roots are entrenched in the water reserve. It has chosen....

Roots that grow into water will normally be just fine, even with bacteria and fungi, but that fact that your trays are encrusted with minerals could mean that the water may be "less than optimal" (toxic, to some degree), and the roots will stop growing when they hit it.