looks to me like a baby sea hare, it's so tiny :D
If you can pick it up for inspection, check if it has a hole in the middle of it's body and then another close to the back, if so, it's a sea hare, and it'll grow into the most OVERKILL CLEANUP CREW MEMBER EVER seriously you will never have algae you'll probably even need to feed it seaweed if you plan on keeping it.