Charcoal like Substance under caulking of Bathtub

Hello. I'm remodeling my bathroom. One of the things I'm doing is reglazing the tub. While I was removing the caulk from the sides of the bathtub, I noticed 1 corner was covered in black. I figured it was mold so I poured bleach in that area and left it alone for 1 day. The following day as I continued cleaning, I cleaned up the black stuff and it looks like Charcoal. Can anyone tell me what this is? Maybe it really is charcoal and some people use this to control moisture. Thanks for your help.
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Put more bleach on it if its mold it will change color, it sounds like mold.
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Hi, Could be just collection of floor dust. If it is mold, BAD/some times dangerous stuff. Wonder if proper caulking was used(one made for bathroom use)? May need to redo the caulking after cleaning. Tony
mark Ransley wrote:

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