The Collaborative International Dictionary of English v.0.48:

Hair grass \Hair" grass`\ (gr[.a]s`). (Bot.)
   A grass with very slender leaves or branches; as the
   Agrostis scabra, and several species of Aira or
   Deschampsia.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hair grass
    n 1: fine-leaved aquatic spike rush; popular as aerator for
         aquariums [syn: needle spike rush, needle rush,
         slender spike rush, hair grass, Eleocharis
         acicularis]