Whoa- you have a TON of headspace. Here's some of my carboys:
I'm guessing that all of that headspace had air that allowed an infection to take hold. I can't tell from the picture if it's mold or something else.
I'd rack out of there right now, from below the crud, and then hit it with 1 campden tablet per gallon (dissolved in a tiny amount of boiling water) and make sure I had NO headspace.