Water/mold in duct work.

Hello. I’m looking for some guidance here. We’ve had a terrible smell for about 5 months and could not figure out what the smell was or where it was coming from. Had and HVAC company out and they discovered water/wet spots under our furnace and said that was the smell. However I recently found this 6 feet from a register where the smell is horrendous! So I’m assuming this is it. How can I get this removed or who do I call? Everyone I have tried said they can’t do it or won’t come out until there is an inspection completed by a plumber to find out where a leak is. When I called a plumbing company they said I have to have a leak detection company out to find a leak first. We live on a slab with no crawl space. We had 2 areas where water was possibly getting into duct work but this photo is on the opposite side of the house near 2 bathrooms and washer and dryer. Any thoughts/ideas would be appreciated.