The Collaborative International Dictionary of English v.0.48:

Conferva \Con*fer"va\, n.; pl. Conferv[ae]. [L., a kind of
   water plant. See Comfrey.] (Bot.)
   Any unbranched, slender, green plant of the fresh-water
   algae. The word is frequently used in a wider sense.
   [1913 Webster]

WordNet (r) 3.0 (2006):

conferva
    n 1: any of various algae of the genus Tribonema; algae with
         branching filaments that form scum in still or stagnant
         fresh water