Help...What is this stuff?

[duraleigh]

If it's growing, it's organic........Clorox is the answer.

Posting a set of test results will probably give a clue or clues to confirm that but my guess is your Chlorine is or has been too low.

[Watermom]

Not necessarily Clorox, but any generic laundry bleach is fine. Most of us here use generic Walmart brand which is called "ultra" and is 6% bleach. No scented stuff.