Some visual effect to be applied universally to any floor that has harmful properties.
My idea initially - make upward going "evaporating" particles, with either color or density being an indicator of just how much hurtful the floor is.
IDK if it is possible to universally match a particle color to the main color of the texture.
Optionally - add popping "boiling" bubbles (from Heretic/Hexen lava with recolors) to the surface of known liquids if said liquid is Damaging.
Another option - apply some sort of special easily identifiable glow to the texture? I'm not sure on how well that will work/look.