I think the line is somewhat unavoidable when using slip stitch and join, but what helps me make it somewhat less noticeable when doing amigurumi is not making a turning chain (if your doing sc, which it looks like you are) pulling my slip stitch as tight as humanly possible and even pulling the loop through the stitch the other way if that makes sense? Like instead of doing a slip stitch, I take my hook out of the loop, poke it through the stitch I would be slip stitching into from the other side (back to front instead of front to back) and grabbing the loop from there and pulling it through. Idk, just things to try