when you say mold - do you mean fuzzy stuff on the betta's body?Or is it growing in the tank?
If its on the betta - you most likely either have a fungus or cotton mouth disease which is a form of columnaris.
If its growing in the tank, not sure what it would be there might be too much of something in the start zyme that is causing the growth?
Loss of appetite
White spots on mouth, scales, and fins
Cottony growth on mouth
Fins disintegrate beginning at the edges
Looks like a small cotton ball
Symptoms of Stress & Disease
Cotton Mouth General Description
Mouth Fungus is so called because it looks like a fungus attack of the mouth. It is actually caused from the bacterium Chondrococcus columnaris. It shows up first as a gray or white line around the lips and later as short tufts sprouting from the mouth like fungus. The toxins produced and the inability to eat will be fatal unless treated at an early stage. Hard to treat, but not impossible. Like any disease best caught early. There are commercial products available to treat cotton mouth. Also change the water daily 20% and use a regular dose of salt in your tank.
I would try treating with an antibacterial medication such as furan 2 IF
this is what your fish has. However if it is in fact ick, then a medication such as quick cure would work.
Do you have a picture of him?