Quora – fungi in multicellular organisms

Quora – What are fungi in multicellular organisms called?

Depends entirely what they are doing there.

They can be working cooperatively with the organism, in which case they can be somewhere on the spectrum of symbiotic associations; or
they can be more on the spectrum of parasitic organisms (that do more for themselves than for the host); or
they can be more on the spectrum of invading organisms (disease organisms), that are more purely there for their own interests at cost to the host organism having somehow managed to evade the host’s defenses.

There is no simple answer.
It’s kind of like asking what do you call a person in your house?
If you invited them there, they may be a guest.
They may be there to benefit you – if they come from the communications company to fix a problem they have remotely detected, or a doctor on a house call (fairly low probability event these days).
They may be burglars there to steal you stuff, or gate crashers to your party.

Without a lot more specific detail, the question does not make much sense.