There is no fix to this aside from redoing the floor ..sorry.. Rubber mat has reacted to the petroleum base of the varnish to cause this discoloration..There are certain ,thin slip resistant mats that specialy made to put under rugs. It allows the wood to breath and not come into full contact with the wood.No cleaner will work..Bona is a great day to day cleaner for wood flooring but it won t work in this case.. GL

Try cleaning it with Bona.

I have light oak wood floors in my kitchen and I bought a mat a couple of months ago it's a rubber mat and I picked it up today sweep and there's a green stain underneath. I had no idea rubber mats stain wood. How can I remove this?