Is there something else that would be more obvious? I might change them to barrels
How about making them look like something owned by the necromancer? A sprite that looks clearly evil.
It could be a barrel, but it could also have glowing red eyes and be standing there….menacingly!
Perhaps something the player could mistake as an enemy rather than scenery. Unless you want to go the opposite route of making lots of things in the world destructible.
Like if the player could keep shooting trees and rocks and stuff, and eventually they'll break and drop coins. Then they'd be in the habit of destroying everything and shooting anything they see.