You can get a command from your lord to grow and harvest the unfarmable raw fungus.
Raw fungus doesn't exist in the vanilla game, so I believe this is a mod bug. I'll move it there.
It exists in the core game. Have a look at
Data/Core/Defs/ThingDefs_Plants/Plants_Cave.xml
and
Data/Core/Defs/ThingDefs_Items/Items_Resource_RawPlant.xml.
Damn you're right, I'm sorry. I forgot about it!
Will fix.
Fixed for the next build. Thanks for letting us know!