Can you get a straight-on, top-down shot of the critter?
I'm familiar with a number of different nudibranchs, sacoglossans, and flatworms; some of them (such as a few Pseudocerotid flatworms) look relatively similar to this, but none of them have that that odd, corkscrew appendage you can see is somewhat separate from the body on the left in the video. Honestly, that appendage has me wondering if you've got an octopus or cuttlefish mimicking a flatworm. (Octopuses can fit through/into any holes that they can fit their beaks into, so it likely could squeeze between the rocks in the tank as, but this would be rather odd behavior for an octopus).
