Probably a spot where water collects and doesn't drain properly causing the mold you see.

Thanks for the response. I have watched it an no water collects on top. Could it come from underneath and cause this?

Maybe the water drains off the roof in that corner, or maybe it's where the dog pees(?) . Just a thought.

What's that thing sticking up from the ground ?

"Could it come from underneath and cause this?"

Yep.

It's interesting the stain is close to the same colour as the wall. Can you take a front photo of the space? From the pool side. Please get a bit more of the house architecture in the photo. Roof line, etc are very important.

And yes, where does the dog like to urinate?

Thanks for all the responses. To answer a few of the above questions: Dog does not pee here. He does his business in the yard. The roof does not drain here. We have watched during rain storms and next time will video but nothing drains from here and water does not collect in this area. This is why we think it maybe coming from below the tile. I would like more info on the organic mold and if the one that had it first was the cause and it is spreading if anyone has any additional information on this I would appreciate it. I will ask the builder to also pull up the tile to see what is under it.

"I would like more info on the organic mold and if the one that had it first was the cause and it is spreading if anyone has any additional information on this I would appreciate it."

Here you go, but unlike me, Fred doesn't give information away for free.

Did you call the company that installed the marble?