Hey Growmie,
Too much water, poor soil drainage, contaminated potting soil, and a lack of light can all cause fungal problems (mold) in your soil. You did right by removing it and using a little peroxide, but if you don't get the airflow issue fixed and the soil keeps staying moist like this, you'll have this problem again. I see you got a new tent, to help with the airflow you can open the vents at the bottom of it to help.