Identified as a 'velvet nudibranch.' It's dark purple/maroon with white strips, lavender underbelly, and flaming orange gills and antennae. I know they don't last long in aquariums, but I was just curious (considered buying anyway). Any info on ID, diet and care?
I'd suggest leaving it in the LFS tank to die. If you buy it, the LFS will think they sell and then buy more.
If we can't keep something alive, we should leave it in the ocean.
(Please note: I HAVE bought things that don't survive well in captivity because I thought, "If I can't keep it, nobody else can, either so I might as well 'rescue' it.")