Indeed wet sand does not move well, 300 mph leaf blower has a hard time. I had a feeling you would say something to the effect. seeing as moss is a collector and holder of moisture.
can you pour water on the moss and collect from the bottom of the rock with the vacuum after an hour or two dry time?
how recently has it rained? perhaps a week of dry weather to give that sand some time to dry might increase the payout.
very cool Idea, keep us posted.