Anybody know what this is?

And should I be scared? This is growing on the front door of my apartment. I'm guessing it's some kind of mold or fungus. But it's growing in an ever-expanding spiral pattern that's kind of cool looking. Unless it's dangerous (or even just gross). It sort of looks like a snail. Hey, I wonder if this is snail gang graffiti! Maybe the Apple Snails found out I was talking smack about them (specifically, using a fungo bat to launch them from my doorstep back into the pond from whence they came) so now they've tagged my home!
I'm not sure what I should do here. Prepare for germ and/or snail warfare, not worry about it and see what happens or just take a sponge and some bleach water and clean it off? Any feedback is appreciated.