Travel Humidor Mold

I recently bought a Xikar travel humidor with the foam lining and the small built in humidifier. I've just been keeping some small cigarillos in there and watering the humidifier about every four days (as per the guy at the cigar shop that sold it to me told me to do) with distilled water using an eye dropper. I water it just until I can see water not soaking in and then take a paper towel to absorb the excess that is not soaked in. I've only had it for less than a month and the other day I opened it and noticed mold growing only on small the humidifier. I took the cigars out and stored them, wiped the humidifier with a paper towel to get as much mold off as I could, and then put the humidor open in the freezer for a couple days. I just took it out to let it get back to room temperature and put the cigars back in.

Is there anything else that I should do to keep the mold from coming back???

Thanks!