"B" movie from '50s or '60s: House with a cellar connected to cavern. There's a fungus that turns people into monsters if they touch it. Several people have been "eaten" or "absorbed" by the fungus.
"The Unknown Terror" (1957)?
Here is a plot summary from IMDb.
A woman leads an expedition into a remote jungle to find her long-lost brother, but instead finds a mad scientist who has created a fungus monster that feeds on the local inhabitants.
Here is the complete movie from YouTube.