Looks like some sort of nudibranch, don't quote me on this but I believe that type of nudibranch feeds on flatworms. @KJ you have any idea? You're good at IDs.
After further research it is definitely a slug from the order Cephalaspidea, which includes bubble snails and blue velvet nudibranchs so I have no clear clue of what it might eat. Slugs often come in on the organism that they eat, and will often be colored similarly to what they eat. I would sump it until you can get a totally positive ID, but it most likely eats flatworms.