The island of Molokai has no beaches which are unofficially designated as clothing-optional. Molokai is the least visited of the main Hawaiian Islands - not including Kaho'olawe which is closed to the public.
This is not to say that you will not be able to find a long stretch of beach that will be yours alone and provide an opportunity for nude sunbathing. On our last visit to Molokai, we visited Papohaku Beach, one of the world's longest beaches. For three miles we saw no one on the beach. If you want a private, secluded and beautiful beach, Papohaku is for you.