Have a terrarium with a fern, would be about 2 months old & have noticed spots of mould on the glass.
Has anyone else had this problem?
The walls are covered in condensation even though I air it out at least once a week.
i have mould in the soil sometimes just wipe it off with a clorox wipe then that with a paper towel and it should b ok
If the opening of your terrarium is narrow you may need to let it air out more frequently than once a week. I also see that you have a layer of sphagnum moss over the soil - this may contribute to trapping more moisture in the terrarium than is desirable. I would suggest removing it.
Check out The Fern and Mossery for more tips on how to make and maintain terrariums.
Here is a link that might be useful: The Fern and Mossery
Lack of decent air circulation is most likely your issue. Stagnant air and moist conditions makes a perfect environment for many molds.