I think any roots that grow into the hydroton would be water roots. That setup is like ‘Swicking’, as I understand it (self-watering system). The roots did suck up the excess water. As long as there is air getting in there (it isn’t constant stagnant water), mold shouldn’t be an issue. You might see algae growth on the hydroton, if it’s exposed to light. But that’s not a big deal.