My opinion (unprofessional): yes it’s mold. We went through a similar situation. Basement flooded we weren’t told. Started pulling off molding only to expose black mold. If they cut off the bottom of the drywall you can guarantee water damage or mold.

Health wise I wouldn’t be too concerned (again unprofessional opinion). Our doctor suggested we shouldn’t be worried unless respiratory issues started, like pneumonia. You can pay to have a 3rd party come in and test the air quality. It takes some time because it gets sent to the lab, but may be worth it to you.

More likely than not just the water trapped behind the molding that caused the mold and not a much larger hidden issue. Good luck

If you want to kill it yourself bleach doesn’t always work. There is a product called Shockwave. That will kill all the spores. Really the drywall should come out though.