Leave it alone. It sounds like you have the proper amount of ventilation.
Mold is everywhere. You are breathing it while you are reading this. Especially mold in an attic - so what? Do you want to remove it from your yard too? The summer heat in the attic will probably make whatever you can see disappear - I don't know how hot it gets in your attic. - then it will recycle again every year. If there is no water intrusion into the area, other than humidity, forget about it.