I cleared a Crimson chasm, and found something I can't find anything about. What is this?

Solution 1:

Looks like just a natural cave wall (a new 1.3 texture, that may or may not only spawn near Crimson). Can you place torches on it? If so, it's a wall.

So, good news: Walls can't spread Corruption / Crimson, so even if this is due to proximity to crimson, its not "infected", just different looking.

The thing about walls, though, is that you can't destroy them from the middle -- you need to work your way from the outside in. Dynamite also destroys background walls, so I'd try lobbing a few sticks into this cavern and see if that blows any holes in the background. If it does, simply continue destruction from those holes, removing as much as you need to achieve peace of mind.