Sorry I wasn't clear...the original line broke from brittleness and was TOO SHORT to exit through the vent baffle and so was dripping inside the fridge compartment. The end plug was long gone along with the broken end. I replaced the line and added the plug you linked to. Although it's hard to digest the price of that little thing, no bigger than a dime.
My guess was also some kind of insect detritus, I just had never seen anything like and wondered if anybody had. Thankfully fridge is working great, although it hasn't been too hot outside.