• (n.) A genus of minute unicellular algae including the red snow plant (Protococcus nivalis).
Pro`to·coc'cus noun [ New Latin See Proto- , and Coccus .] (Botany) A genus of minute unicellular algæ including the red snow plant ( Protococcus nivalis ). 
Protococcus is a genus of single-celled Algae. The British species Protococcus viridis forms a yellowish-green stratum on trees, damp walls and shallow pools.