my guess is some honey has leaked and fermented in the channel. Best thing to do is to ensure the hive has the correct tilt- and then periodically turn the little cap around so that the hole where the bees can lick up honey is kept clear. You can poke a small twig into it as well if it is blocked with propolis. Then get a long bottle brush (not the plant- an actual bottle brush). Get a bucket of warm water- dip the brush in and run it in and out of the harvest tube- dipping it in the bucket of water to rinse it each time. Then leave the plug out overnight so the drain tube can dry (you can stick a ball of paper or cloth in there to prevent ants or bees crawling in but allow it to air out). Then harvest assuming it has stayed dry and no more honey has dribbled in.
I often clean my tubes like this a day or three before I plan to harvest. Many times the trough is squeaky clean but sometimes there is a little honey residue- or some wax particles and possibly a dead ant or some such.
Then when you harvest your flow frames put a good fine strainer in line so the honey is strained as soon as it comes out. that way is ready to be jarred immediately.
That channel cannot be accessed by the bees- and is kept very warm and humid all of the time- so any honey that may leak there has a potential to ferment. As the frames get older it is more likely for it to become a little mucky. At the end of this season when you remove the flow box before winter you may want to inspect the frames and check the wires- especially if any certain frame has demonstrated this issue more than once.