Maybe, but is it active or old? It could have been on there when the stud was installed.

Doesnt matter what it is, clean it up. some 1 to 10 ration bleech water in a spray bottle will kill everything. Wipe it on with a rag and let it air dry. We just ripped out our bathroom to studs also. Not sure what some of the stuff was, but we hit it all with bleach water.

Careful not to rub your eyes also. Gloves help as well.

bleach won't kill mold on porous surfaces. use vinegar or hydrogen peroxide.

Concrobium for mold. It's usually available at the big orange store.