Evil Polychaete

[Youkai]

Ever since I started my aquarium, I've had issues with the slow but steady disappearance of invertebrates.
Snails are usually first, then small hermits, then even large hermits. Even peppermint shrimp. My abalone.
Fish do just fine. The only fish are, infact, seahorses and pipefish.
Corals thrive...however, I've noticed that it seems something is going for the zoanthids.
Water parameters are all normal.

Then, just the other day I walked by my aquarium and noticed a huge amount of slime. Normally, when I walk by my tank, it doesn't look like ectoplasm is billowing out of the rocks. This caused me to raise an eyebrow.
Upon investigation, I found an unusual worm that seemed to be attacking a hermit. Instead of 'bristles' (like bristleworms) it had many legs. It has a flattened head with what seems to be very simple eyes. It was going inside the hermit's shell, and had covered itself and the crab in this slime.
It escaped into the rockwork.

If anyone can identify this critter, or has any suggestions...let me know.