That line is the starting/stopping point of the perimeter loops. It cannot be avoided. The loop has to start and stop somewhere. You can see this in the Layer View. Use the slider at the bottom to scroll through a particular layer and see how the print head will move.MatterControl does it's best to put that seam in a place where it will be well hidden. However in the case of a cylinder there are no good places to do that, so it opts for a single line running down one side of the print. Other slice engines (like Slic3r) give you the option to randomize the starting points of the loops. This creates small blemishes distributed randomly through the print. In some cases this might be preferable.In some cases it might also be appropriate to use Spiral Vase Mode. In this mode, only the outer perimeter is printed, making the object completely hollow. The print head moves up continuously as it goes, laying down plastic in a helical shape. This means there is no starting/stopping point for each layer and the seam is eliminated.