Masters of Disguise
Since the other sea creatures that they choose to prey on are also dangerous and deadly, they have to become masters of disguise. That also means that they disguise themselves to humans, which could be very dangerous. It's totally possible to not even realize the danger that's lurking!
They Float Belly Up
The slugs are pelagic, which means they float belly up. The blue on their underbelly helps camouflage them with the blue color of the ocean. They stay afloat by swallowing air and holding it in their stomachs.