Looks lik a nudibranch to me. and is that a green lettuce nudi just above it in your second picture?
Watch where the "unknown" likes to hang out. If it's anything like the lettuce, it'll eat the bryopsis.
it's a sea hair (anaspidea). that dark stuff it let out is a type of poison. in larger quantities its purple. leave it alone and it will be fine, but i have seen them get about a foot long, and there poison could be a problem for the tank.
Sorry mate, I believe the common name should be "seahare" not "sea hair". It's called sea hare from the fact that when they reach maturity, their ability of breead like bunnies.
These are herbivores. Their only defense is the purple ink they secrete to help them deter enemies.