What is growing in my car?

I drive a pacifica and have stow and go seats. I lifted up the floor to clean under it yesterday and found this.

-I live in PA, so salty roads. -Over a year ago 1/2 bottle of listerine spilled in there while on a trip. I did clean it up, but maybe not enough?

The first 2 pictures are from when I found it. The 2nd two pictures are from this morning. I have used Mr. Clean foaming spray (used a drill with a brush to scrub), peroxide (used a drill with a brush to scrub) and a rug cleaner. It keeps growing.