A little mystery in Monument Creek

Pat and I went for a walk in Monument Valley Park a few days ago after work, and I couldn't help but notice this large boulder in the creek was covered with white stuff. I tried to get a decent picture but it was pretty far away -- there's quite a steep bank and the rock was in the middle of the water. But as you can see, the entire top of the rock is white. None of the other rocks in the stream had this problem. I know it isn't pigeon poop (that would take a LOT of pigeons), and duck poop is green. My best guess is that the rock had been submerged for most of last year thanks to abundant rainfall, and was subsequently covered in algae, which was later exposed and then bleached white by the sun. It's a decent theory, anyway! Any other ideas? Enlighten me!