It's a nasty, one of the bad polyclad flatworm. It will
decimate your snail population,
That is a cool looking polyclad worm
Not sure what they eat, seen one in a nano tank that disappeared and reappeared a couple of months later. Nothing died in between. Could not see what is being eaten. There was snails and hermits in there. Some research suggest detritus.
Another Critter - flatworm or what? - Marine Aquariums South Africa