As you can see the result is no good. The polish is insufficient. Even the bulk imperfections and discoloration from the mold damage haven't been taken out although I have polished thoroughly with good pressure. Should I have tried lower grit than 400,or bring a professional to do the whole room?(I'm trying to do only the part which got damaged)
Your first mistake was using the diamond pads. Part of the discoloration is stone dust being embedded in the concrete filler. You had a mold problem or staining from food/drinks being spilled. You should have started out with a non abrasive concrete / grout cleaner. You could have even used a mixture of beach and water. Try doing this and just work on the cement/grout filling. Once that's cleaned up use a buffing pad and polish on the affected stones.