What on my shower floor and how do I permanently remove it!

[johndaly24]

Good morning all I have a new home that the builder installed this river rock style tile on my shower floor. After a few months this black stuff started coming out so my wife used bathroom bleach products to remove it. It goes away fine and then a few weeks later it comes right back and in the same places. I have thought it was mold not sure why as shower is cleaned weekly. I have tried Concrobium Mold Control Spray, but that did nothing. We are at our wits ends. Hoping to get some guidance here. Thanks in advance

 

[Kastar UK]

Good morning all I have a new home that the builder installed this river rock style tile on my shower floor. After a few months this black stuff started coming out so my wife used bathroom bleach products to remove it. It goes away fine and then a few weeks later it comes right back and in the same places. I have thought it was mold not sure why as shower is cleaned weekly. I have tried Concrobium Mold Control Spray, but that did nothing. We are at our wits ends. Hoping to get some guidance here. Thanks in advance

Is living deeper within the structure of the grout ? , try remedy those bits with new grout and seal ?
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