Black Mold or Mildew???

[Joey Hampton]

I was going to finish caulking a bathroom that wasn’t caulked correctly, and so I was cleaning along the edges of the plastic insert of the shower, and noticed chunks of black stuff kept falling. So I started pulling the old caulk away and noticed it had all this stuff behind the caulk. I was worried it may have spread up behind the back of the plastic insert, along the wall??