Mold Behind Tub Surround
This is mold that was behind a plastic tub surround and who
knows how long it has been growing here. As you noticed the mold is
growing at the bottom area and not at the top or above the tub spout.
This is where most the moisture will be. The lower area whether it's the floor or the bathtub will suffer the most damage.
This is a great example of something you will not be able to see since you don't have the x-ray vision. But you can look for clues.
For example: look for loose plastic tub surrounds and mulled around the bottom corner where the plastic tub surround meets the bathtub. This area should be caulked and the caulking should not be loose or missing.