It’s being used as a participial adjective here. I don’t think there’s much semantic nuance in the distinction between “melted” and “molten.” It comes down to common usage. “Molten sand” sounds more suitable to my ears, but “melted butter” also sounds better than “molten butter.” Odd, I think it’s part of a trend to replace the use of the past participle in some adjectival contexts. Maybe melted/molten is just an incomplete transition.