Andreas, I've purchase several
greeks that have that exact coating.
I think I can even name the seller judging by the picture.
I think when you get the coin in hand you will find soft clay
that has been waxed over. Careful picking with a soft tool ( toothpick)
will lift the clay of in large chunks, the wax tends to keep it from
crumbling. Do NOT soak it first, that will soften the
patina and smear
the clay. They tend to be easy to clean using a scope & pin only.
Larry