Crimson Crust Wall

You cannot harvest it. Breaking the wall in an Underground Crimson drops nothing, so the item only ever comes from a crafting station, and from there it is normal building stock you can place anywhere in the world.

Crimson spread rewrites cave walls one for one as it moves. Crust is what a Worn Stone Wall becomes, Mottled turns into Teeth, Fractured into Blister and Stalactite into Scab. None of that reverses on its own. Pulling a cavern back out of the Crimson means a Clentaminator and Green Solution, and no other tool will do it.

The world's own copy of this wall does nothing to stop enemies appearing in front of it. Yours does. If you have carved a farm or a fight arena into the Underground Crimson, re-walling the whole interior with crafted wall is the step that makes it controllable.