The 38D is semi key because it is one of the harder dates/mints to find. That particular nickel has a mintage of a little over 5 million, and is worth about a dollar or so. I would leave it as it is, cleaning coins is usually not advisable, but a soft cleaning of soap and water may help clean the grime away. Hope this helps!
first year of the "new" jefferson nickle 1938 D -- was the buffalo nickle before that -- a small number made 5 million --so if one collects "jefferson nickles" -- it is a hard one to find to "complete the set" -- key date /mint mark are coins are often very very hard to find (often made in very very low numbers--very rare and very hard to find - thus they tend to have "collectors" value )--semi key date /mint mark are are somewhat hard to find (made in fairly low numbers-- they have some "collectors value" )