The Collaborative International Dictionary of English v.0.48:

Sea moss \Sea" moss`\ (?; 115). (Zool.)
   Any branched marine bryozoan resembling moss.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sea moss
    n 1: sessile aquatic animal forming mossy colonies of small
         polyps each having a curved or circular ridge bearing
         tentacles; attach to stones or seaweed and reproduce by
         budding [syn: bryozoan, polyzoan, sea mat, sea
         moss, moss animal]
    2: any of various red algae having graceful rose to purple
       fronds (e.g. dulse or carrageen)