To be honest, it's likely just bio-film. It happens in ALL filters or tubes through which cycled aquarium water flows. This is why when you hear people say bacteria will grow "everywhere", this is what they are referring to.
It will eventually "slough" off and end up possibly floating in your tank until the filter catches it again. It's NO BIG DEAL, and short of cleaning your tunes every few months there is nothing you can do about it. I don't let it bother me in the slightest, and neither should you!