In world?How do you teach the players (or characters) that, in world, yellow stuff on the floor will kill you without making the yellow mold a pointless obstacle? If it's too deadly, characters just avoid it. If there is a steep, but non-lethal cost to stepping in it, then you open the players to making a hard choice about whether to do so.
Maybe some characters die. Others survive, and now know about yellow mold.
Maybe there are encounters with various kinds of mold prior to this, so the players are leery of all dungeon-mold.
Maybe other characters who don't die from the encounter might write down "dungeon ecology" books, or read them written by other explorers.
A chance not to die the first time? Maybe the first time they find the mold there is a skeleton in it. Maybe it is along a route that isn't protected by the Goblins and they should wonder why the Goblins aren't protecting it.