So I beat the Golem and finally decided to buy the Clentaminator to clear the world of most of the more troublesome hallow and Crimson.
While clearing the chasms I found this background area that doesn't seem to budge to anything. The Clentaminator doesn't purify it, the Pwnhammer doesn't smash it, and the Pickaxe Axe doesn't seem to have any effect on it. And while I did try searching for it on the web I didn't manage to find any info.
What is this thing?
After posting, I managed to find another one under the other Chasm, and they are destroyable by using bombs (that destroy tiles) and dynamite. So I guess they're just background tiles as Sable Dreamer claims.