Cause And Cure For Asplenic

I am 59, female, 150 lbs. and in good health; I am asplenic. For the last 5 years or so small areas of skin on my trunk and legs have extra skin that grows over the patch. The patch is darker than the surrounding skin. (They are not moles because they are flat and never change color or shape.) Sometimes, when I shave, the extra skin is removed and the patch regrows. What are they and why am I getting these? FYI: No history of skin cancer; no sunbathing or tanning booth use since 1993. Is this just something that happens when you age?