We just removed some molding from our floor only to reveal some kind of black substance that looks like dirt. We are not sure what it is. See the following photograph.
That appears to be black mold affiliated with a water leak or a flood. If a flood you need to have mold mitigation done by a qualified mold company. Drywall removal will be required as the mold you see is coming from the mold on the back of the drywall. Mold loves cellulose as a food so the paper backing on the back of the drywall will be completely covered in mold. If a flood situation it is recommended to cut a minimum of 1 foot of drywall above the noticed mold. However the paint can hide the evidence of mold and can be 4 foot or higher that needs to be removed. After removal in a flood situation it is recommended to use a disinfectant grease removal soap and scrub all areas affected to include the entire home. I clean a minimum of 1 foot higher than flood level. . I use Pinal lavendar sold at home depot as it has a nice smell. Then dry the home or area to a maximum of 50% of wood moisture measured with a moisture meter. You can buy this at home depot for about $30.00 After reaching 50% moisture then spray all wood down with anti microbial mold killer. Soak it down completely 3 times over 3 days no fans, no dehumidifiers. After 3 days then turn on fans to create a circle of wind in each room leading to a dehumidifier in each room. For entire home floods, you will need more than 1 dehumidifier. Recommended is 1 per room. So in many cases a mold control company is cheaper as we own equipment. It is also recommended to have a Hepa Air scrubber to remove airborne mold spores. After the wood reaches a level of 10% or less, you can install new drywall and insulation. However I require that my customers continue to dry under normal AC or heat for 1 week after certification. In cases of clean water damage like fire or water leak you can paint the studs with antimicrobial paint. In cases of flood damages, I do not recommend the paint unless you allow the home to dry out 6 months or more prior to painting and closing the walls. Flood bacteria takes more time to allow the antimicrobials to penetrate to core . And if you paint to soon...you will have structural failure from dry-rot bacteria (Fusarium sambucinum) prematurely.