It looks like the encrustation is mud only. If so, you can take the mud off by soaking the coin in warm, soapy water for a day or so and wiping it carefully with soft cloth (I like to use old baby clothes). Don't start wiping until all hard parts of mud have fallen off, or you will make scratches on the coin. Soaking it longer is preferable. Dry thoroughly after this treatment, preferably in sunlight.
Peter