Orange Bloodroot

It hangs off the underside of dirt rather than growing on a floor, so the hunt is a matter of watching ceilings while you walk. Dirt thins out the deeper you go, which is why the upper underground pays better per minute than the cavern layer for a dye run. Swinging a sword through it does nothing at all. Bring a pickaxe or you will walk past a dozen of them.

On the map it draws as a small red mark, identical to dripping lava and close enough to a life crystal that you will dig out a few disappointments. A Spelunker Potion is the fix, since it lights the plant through stone and turns a slow ceiling search into a walk down a corridor picking things off.