It's blue water leaking out of the dump chute of the waste tanks.
Most tanks have a valve at the top of the chute, a seal at the bottom known as the lav donut, and a cap at the end. In normal servicing, the lav driver opens the cap, attaches the hose from his truck. The hose has provisions to remove and install the donut while it's connected to the aircraft. Once this is done, there's a pull handle that opens the dump valve at the top of the chute that allows the contents of the tank to gravity drain in to the truck.
If any of these components fail, it could cause a leak, but what normally happens is that the driver forgets to re-install the donut, meaning that the dump valve and the cap are the only sealing elements. If the dump valve sticks open for some reason, the cap at has the entire job of keeping the fluid in the tank. This also results in a nasty surprise for the next lav driver. The next time you hear someone say that there is a whole school of "brown trout" under the aircraft, you'll know what they mean.
Worst case scenario is a leak that results in a large chunk of ice that breaks loose when its mass gets large enough and causes damage to the aircraft or things on the ground below it.