Cleaning of sea melon seeds: put it in light salt water and stir it clockwise (don't disturb the direction), then change the water and stir it the same way, so that the sand can be washed off after several cleanings.

Or soak in clean water and put iron in the water. It must be iron, knives and forks, or nails. This is how shellfish spit sand. Change the water twice and soak for one night. It should be clean.