@Belinda I think your best bet is to ask @Woodbridge Environmental Tiptophouse.com I have tagged him in the hopes that he can best direct you. Mold is a tricky thing and the risk has to be assessed as to whether or not you need a professional, and the advice on treatment has changed in the last couple of years.
what is feeding the mold ? most mold occurs in the lower portion of a house where condensation feeds it . i reccommend a new start or taking them out and thorougly clean them ansd then put them back. eliminate the source by dehumidification.it cannot grow if it has nothing to feed it
Replace the duct and get a dehumidifier installed. Call us at 757-717-2653 for estimate. Haskells Heating & Cooling
clean the ductwork with bleach