The one thing I would warn is that if the things get more powerful as time goes on, you maybe only make that happen if the player is nearby, so they don't go explore the other side of the dungeon, not even realizing your fungus is there, and come back to find rooms full of mature superfungus.
Of course, if your levels are small enough, this doesn't really apply.
I played a game once (I think it was Crawl with its Oklob plants?) where there was some kind of reproducing monster and it ended up taking over a large portion of the level just because I went the other way. Annoying, especially 'cause even if I knew that it did that, I had no idea it was even there.