By definition, plankton is something that floats in the water column, without any means of propulsion. Nekton is something that floats in the water column, but can propel itself.
Now there could be things that create plankton/nekton in your livesand, such as polychaete worms and copepods, etc... But if it's living in your sandbed, then BY DEFINITION, it's not plankton.
Cheers
James