If it is flattened and not round I would take a guess it is a flatworm (phylum Platyhelminthes). These are often associated with bad things, but not necessarily so. Let him burrow into the sand and then just siphon the sand out around him. Then you can stick him in a jar to get a better pic
. You might have a real jem there, some flatworms have some awesome colors and your specimen looks pretty neat