Even if they are, I don't know much about them. I've heard rumors to this effect from one lady elf who claims to be of their line. They probably have some magical means of coming to earth, but I'm not sure what that might be. Shadow Elves I've never even heard of. I don't know an elf who fits neatly into either category, I'm afraid.
Feral Elves sound like fun.
I wonder if they're dangerous. I can only guess. Sea Elves are fairly common where I'm from. They live underwater, usually ocean or seawaters. They can breathe air, however, so I suspect that the gills are a later development.