If this stain is not "sticky", i.e. doesnt not leave dirt on your fingers and causes stains elsewhere: this is your cameras first scar and shows, it is yours in a case of doubt.
I take care of my equipment, but as long as it doesnt take effect on IQ or functionality, I wouldnt care.
1D MKIV, 1DX MkII, 5D Mk IV; lot of lenses, flashes etc
Fuji X100s, Sony RX100 III,
Fuji X- XT1, X- Pro2, XF 23 1,4, XF 18- 55 2,8-4, X- T1, XF 16/1,4, XF 55- 200, XF 56/1,2, XF 50-140, XF 100- 400