What do you call this effect in 3D printing and how can I remove it? I have a Bowden extruder in my printer setup. Everything works great except this effect which must be related with retraction and is generated on Z layer change:

  

The model itself, is printed in one layer perimeter / outline shell, with 0.25&nbsp;mm layer height with the following settings:
What you see on the outer surface is called "zits and blobs".

These small imperfections you experience are "zits" (larger ones are referred to as blobs). As the extruder needs to start and stop as it moves around during a print, it is difficult to create a seamless joint, so the over-extruded filament represents the location where the extruder started (or ended) printing a section of the outer perimeter of your print model. Sometimes it returns to the same spot in a single extrusion run, in other cases the perimeter is constructed of multiple sections.

It is possible to do something to minimize the effects depending on the slicer you use, but the general solution is to prevent too much plastic being deposited at either the start or the end of that seam. E.g. incorrect retraction settings may cause too much plastic to be extruded at the start, and pressure build-up in the nozzle may cause an excess of plastic to be extruded, both lead to the imperfections you experience. First you have to find out which of the two effects is happening with your prints. Once identified, you can play with settings like retraction, priming, and coasting to counteract on these imperfections. A more detailed description can be found here.



Edit:
Please read the addition posted in the comment by @Trish; the comment describes that this may also be related to over-extrusion!