something is very wrong here, you should have an abundance of roots overflowing in the amount of time youve stated the plant has been vegging, your probably experiencing root rot due to poor aeration of your roots, your rockwool is probably oversaturated if mold is growing at the top, i start my plants in rockwool cubes and once they have rooted i make sure the top of my medium is kept bone dry, the mold starts at the bottom and works its way up, if its dry at the base of the plant the mold has nowhere to grow, your also experiencing mold because you have lack of propper ventilation, high temps and high humidity, get a fan, get more air exchange, and if the budget allows get a dehumidifier, if the plant seems to be growing steady and you would like to keep her, by a spray bottle, make a mix of original bleach not scented, 90% water, 10% bleach, mist the mold lightly, i hope this helps you