The slave is the cylinder thing held on to the LH cover with 3 screws just forward of your left boot.
Yes damage where the seal seats. The OEM cycliner is just soft aluminium and can get easliy scored with dirt or ridged by the piston which is often not a very good fit. Corrosion is not generally an issue but the castings sometimes have pinholes which don't help them seal.
If you strip it and replace the seal, bleeding can be a real problem. Once you start getting fluid to come through it purge the slave cylinder by loosening the bleed screw, pushing the piston all the way in, tightening the bleed screw and then releasing the piston. Air usually gets trapped at the top banjo as well. This can be removed by tilting the bike over so that the reservoir is higher than the banjo or by loosening the banjo bolt slightly, pulling the lever to expel the air and tightening the banjo again. Use a rag to catch any spilt fluid and if yopu gat any on the paintwork wash it off immediately with plenty of water.