[1.2.3] Hidden, intangible mold boss in Ruined Dungeon

[BrainWeasel]

Stuck trying to open up the vault of the Ruined Dungeon, as one of the six key enemies is hiding in a way that makes it untargetable. As you can see from the screenshot, I dug out quite a few walls trying to find it. It's a mold, so I can only think that it messed up a Slime Roots somehow. There is a 3x3 set of tiles in the room shown in which it can apparently see and attack my character, but I cannot counterattack as one normally can against invisible or stealthed enemies.



Save file: https://www.sendspace.com/file/e7ga84