The Collaborative International Dictionary of English v.0.48:

Sea mat \Sea" mat`\ (Zool.)
   Any bryozoan of the genus Flustra or allied genera which
   form frondlike corals.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sea mat
    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]