why is the soil moldy?

Hi guys, the soil is moldy and I don't know why. So far the humidity was around 60-70 and after molding 40. What do you recommend?

Air movement, add fan/fans.

Warm wet humid conditions perfect for mould and fungi to grow. I sometimes see it when germinating seeds straight into soil, if I haven’t let the air in and out the propagator for a few days.

On top of what Cherry already said about airflow, how much are you watering an how often?