That is most likely the krausen in the tubing that came up when the beer was in the most active part of fermentation which is why there is some small traces in other parts of the tube. As long as the CO2 is still escaping you will be fine, I would get a replacement tube just in case it does clog. If it clogs then take the replacement tube, sanitize it, and replace it with the clogged tube.
"Relax, dont worry, have a homebrew." -Charlie Papazian
"A friend with yeast is a friend indeed."
"Beer is proof that God loves us and wants us to be happy." -Benjamin Franklin