Simple Green is citrus based, and might have an effect on what might be citrus oil residue.

Or white vinegar? Test a small area first

Here you go:

^^^ This works because baking soda is a mild abrasive.

I don't have Simple Green, but I tried rubbing one half with lemon juice and the other half with vinegar. Neither one got much of the stuff off, but vinegar seemed to work better. I covered the affected area with strips of paper towel soaked in vinegar and left it for a few hours; that worked better, but still didn't get all of the stuff off.

I think I'll try soaking overnight, followed by rubbing. I'll report back when I have results.