Usually, I don't bother trying to clean the skin. It's normal for the skin to discolour where your hands contact it. It's loads of dead skin cells that shed from your hands and end up getting pounded into the drum head. If you feel you must clean it, don't use water. Instead, try wiping it down with some methylated spirits (denatured alcohol).
If it is grit and oil and dead skin(resembles wax) i'd scrape it with something like a credit card. Usually get most off. Can scrub with a slightly damp towel once most dirt off. If any dampness remains dry with dry towel and leave drum sideways in dry warm place to dry.
better to clean it imo as if you leave the drum for a long time not played mold can grow on the dirt and oils from the hands that played the drum. Plus if you have any hand cuts the dirt can get in.
If something has absorbed into the skin it can't be cleaned.
Depending on the tightness, wetting a skin can break it so be careful with any water around while cleaning.
If this are only dried out hand residuals from playing (skin cells & grease), you can rub it off with slightly wet and than dry cloth. Sometimes rough dry cloth alone will do the job. If both does not help, very fine (1000 grain) sand paper could also help. Note: The sand paper procedure is suitable only for dirt and stains which are applied on and are on the skin but not impregnated deep into the skin. Skin can be damaged with excessive sanding.