It’s just a monster for F&B’s world. This is why I shouldn’t have too much time on my hands. As always, I’m open to suggestions.

As you may know, ghosts and poltergeists appear when people don't go onto their afterlife, either because Death felt nice or tied with them in a game for their lives. Ghosts are basically thoughts, so they can't influence reality. Ectoplasm is formed spontaneously around those ghosts. Ectoplasm is this glowing stuff that floats like a gas, feels like a liquid and can hold things like a solid. The ghosts can shape this ectoplasm into new bodies, which can sort-of interact with reality. Are you with me so far?
Anyway, nature abhors a vacuum: so obviously, creatures had to come along to eat that ectoplasm. This is what the ectoplasmic hunter is for. They float around, searching out ghosts and then eating the ectoplasm shell. This doesn't hurt the ghost, but it takes a while to grow back, which means they can't do anything in the meantime. Ectoplasmic hunters are made of ectoplasm, so they can pass through walls and become invisible and stuff. They have four eyes that they can use to see through walls and the tendrils find minute traces of ectoplasm. Ectoplasmic hunters are very good seekers and are sometimes used by paranormal investigators to find ghost (they tend to eat those ghosts or escape their leashes).