Excessive use of humidifiers can cause mold growth in a room.
But when you use a humidifier in moderation when needed there should be no issue of mold growth but if you do notice mold growth you need to get rid of the mold and cut back on the use of the humidifier.
I only use my humidifier at night or during the day when I really need the humidifier to add moisture to the air such as in winter when the air is dry because the furnace is on and heating the house.
The furnace dries the air out and then can make me cough and lead to nose bleeds in myself and family so I use the humidifier to add moisture to the air.
But I don't run it all day and only for a few hours at a time to keep too much moisture from building up and prevent mold growth.