Actually its spatter… no slag on a mig weld, especially stainless (no silica to float up from the weld like copper coated wire). Spatter is caused by a slightly off tuned wirespeed/heat setting on the machine usually, and even when its tuned great it will still make some spatter, if he really cared about it they would knock right off the pipes with a baby wack with a file or a gasket scraper. but who really cares? shit will never break I will put my nutts on the line to prove it.