I wish I knew what was causing it? My environment seems fine.
Dry conditions, ideal temperatures and an indoor environment. That seems to defy everything I've learned about mold so far. I've only seen mold indoors once, and those were autoflowers taken from outside.
I noticed the scarring on this picture. Outdoors these are the kind of places where mold starts spreading. When the mold is inside the trunk there's nothing you can do but watch the plant get eaten out from inside.