I was 1 of the 4 who voted Au. 58 got involved around the house here and for some reason thought I had stated so in my reply. However did not reply until now. I do believe that this is a slider low MS /Au 58 . That green stuff as well as carbon spots are famous for leaching out of a nickel after slabed. I have a few Buffalos that both have shown up after grading.
I think so too. It reminds me of the Indian Head @C-B-D had that went from Anacs AU 55 to PCGS MS 64 if I recall correctly. And that spot did draw my eye too (it would bother me a bit if it's too prominent in hand...but not if the coin was free ).