You can go Berghia Nudibranchs, but in your situation, kill them off with boiling RO or tank water (or joes juice, but water wont screw your params like alk if you use too much) and get a couple peppermint shrimps for just in case. The shrimps are hit or miss, they wont always go after your aiptasia, but they are fun to watch
About your LFS, did you bring a picture in to show them or did you just describe them? Point something out that was similar in their store? If they were jerking you around I would go elsewhere, but maybe they just misunderstood? Those are FOR SURE aiptasia. Oh yea, and that crap on your rocks/sand IS FOR SURE detritus. blow it off with a turkey baster/power head only right before you do your WC. Even better, if you can suck the crap right off the rocks and sand during the WC, that might work better for you. Might want to think about bumping up that cleanup crew also, what do you have currently as far as snails/crabs? Oh, and what size tank/bio load?