they sell OMRI sprays at Depot or Lowes for mildew, that or a diluted h202 water mix. cut all effected leaves if the sprays dont work. lower humidity and get more ventilation in there. a sulfer burner works great if you cant find a spray.
I would definitely get the spray and apply it asap and if that doesn't work, chop the affected areas. They will continue to spread. Good stuff Trillions. Take care and be safe. Keep us posted.