Mushroom or mold (6)
I believe the answer is:
fungus
I believe this is a double definition.
'mushroom' is the first definition.
(thesaurus)
'mold' is the second definition.
(thesaurus)
(Other definitions for fungus that I've seen before include "One as yeast, mushroom" , "Mushroom, for instance, is one" , "Kind of plant, mushroom, toadstool etc" , "Organism such as a toadstool" , "A mushroom, toadstool or mould" .)