Ectoplasm

Dungeon Spirits only exist after Plantera is dead, and they appear when other dungeon enemies die rather than spawning on their own. The farming loop is therefore just killing steadily and letting the spirits come to you. The Hardmode dungeon hits hard enough that Hallowed armor is the sensible floor, and one trip will not cover a full Spectre set. Plan on several.

Leftovers are never wasted. Each piece sells for a lot, so a long dungeon session pays better in coins than most farms available at that stage even if you never craft a bar with it.

The name comes from spiritualism, where ectoplasm is the residue a ghost supposedly leaves behind when it manifests or is destroyed.