Those balls were usually steel ball bearings with a synthetic rubber wrapped around it. They gave the entire mouse some weight which made the mouse feel better to use. You could clean them with something like soap, but you'd have to be careful not to use anything that messed up the rubber. Some people cleaned them wrong, which caused the rubber to become more sticky and thus get dirty sooner. You'd also risk the rubber becoming harder and not sticky enough, so they would slip a lot. They were basically a pain in the ass and I'm happy we've moved on from that.