Tub is clogged. Can't remove Kohler overflow plate | Terry Love Plumbing Advice & Remodel DIY & Professional Forum

Hi - our tub is clogged and I was trying to see if I could remove the overflow plate to see if I can snake out any build up from there. It's a Kohler K7213-CP and has what appears to be a hex screw with a good deal of hard water calcification. I tried every wrench I had but nothing comes close to working on that screw. Any ideas of how I might get it out?

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