I don't have an AC, but the Penguin filters have a similar tube. I have a long flexible piece of plastic with a hole in one end. I put a wad of filter floss or a piece of cloth in the hole, then pull it through the U tube. A weight on a piece of string would work as well as the flexible plastic- you just need to get something through the tube to hold on to so you can pull the floss through. It scrapes out all the built-up film, lickety split!
(I typically don't do this more than a few times a year, though! The Penguin tubes are opaque, unlike the AC tubes, so they don't grow much algae/gunk.)