Nice coin! I think you'll find that those green deposits very hard, almost rock hard. They are not affected to any great degree by distilled water. Leaving the coin in a distilled water bath over many days, even weeks, might loosen some of the accumulation, but the surfaces will be uneven though less so. There are physical means to reduce the deposits, but this approach requires lots of experience and judgement.
After seeing what your friend did with that Antoninus Pius "Tiberus" coin, I think I will always vote "Clean!"
