Airlock full of crap - Hard Apple Cider, 1 gal

So I'm brewing my first batch of hard apple cider. Everything seems to be going well, except for one of my gallon containers. I'm using English Cider Yeast (WLP775). I split one vial (for 5 gallons) between 3 gallons of Cider. 3 of the 4 containers, I used Yeastex nutrient in -- the fourth, the one with the dirty airlock, I did not use Yeastex in. Now the airlock is packed full of sediment. Any ideas why this happened, and how I should go about cleaning it? Thanks!

You had a vigorous fermentation so it pushed krausen into the airlock. Clean it out, sanitize it, and fill with your liquid of choice. The cider will be fine.
In the future you can avoid this by using a blow off tube.

I'm still new to this so I can't speak to the why, but if fermentation is still really active you might want to add a blowoff tube. Otherwise, rinse it out, sanitize and put the airlock back on with some vodka or a StarSan mix.

Thanks. I cleaned out the airlock and replaced with new water. Gonna keep an eye on it and hopefully will not need the blowoff tube at this point.