have a biopsy on it to check for cancer.
they can scrape it to see if its fungus.
try neem oil on it too.
i also found this:
Seborrheic keratoses occur with aging and are benign, waxy, warty-looking growths that can be yellow, brown or black. They occur on the face, scalp, trunk and extremities and are easily removed, usually with liquid nitrogen.
http://www.gidonaesthetics.com/mole.htm
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