I know, it was the very same headache the very first time I approached the topic 
the big “difference” I felt between printing Clay and Plastic, is that with Clay the extruder -usually- squeezes the clay snake that is being extruded
this way the layer you are printing usually becomes larger in size than the diameter of the nozzle, because the material “has to go somewhere” → you decide the height of each layer in such a way the extruder itself becomes also a “scraper” that splats the clay while it goes, knowing that the level of the previous layer was exactly the trajectory of the previous pass, because the nozzle also splat that
[edit: that is also because Clay is something of a more “stubborn” material than molten plastic, you need to force it to weld and settle on the previous layer so you need to apply some force and shared surface, and the splat helps for all of that]
I have this ancient video where the splat is very visible
when you get to this point, you’ll see that the material that is being extruded sort of “pushes” a little bit on the previous layer, that deforms a bit and has a sort of rubber-gummy-pushback effect (the first time I saw that with my eyes I was mind blown, as the printing of a layer somehow visibly deforms the previous ones for a split second, and the print looks “alive”)
and if you hover the extruder mid-air, it will just extrude perfect vertical same-diameter-as-the-nozzle snakes of clay of course
so the combination of these two things, that the top of the nozzle “scrapes” the clay, and at the same time the extruded clay doesn’t find a bottom surface that gives enough resistance to be squeezed in-between, produce by itself these kind of swellings here
a planar X/Y movement of the extruder, that goes slightly off-path than the previous layer is already enough to create evident surface patterns
then, the more you move off-path, you start printing over the void, there’s no bottom layer anymore to hold and squeeze in-between the clay being extruded, so it becomes the very same shape of the nozzle: a perfect snake
regarding slicers, creating G-code that you can use to 3D print Clay is really easy in GH, 100% feasible using just a small portion of vanilla components!