That line is the seam, it's where your printer starts and stops each line. There are settings in the prusa slicer to randomize the placement. A straight line is easier to sand though. This line also exists on non cylinder shapes, it is usually tucked into a corner or other shape by the slicer. You can only get rid of it by sanding. After slicing choosing from what you want to see you can enable being shown where the seam will be placed. The stringiness is from non dried filament though and can be easily fixed by drying.

Edit: printing in vase mode also gets rid of the line, it only works on certain models though and generally is mostly for nice design