Hopefully someone on R2R or the @Reefsquad can help. Otherwise realize there are multiple thousands of different nudi's. If it helps, they tend to be the color of the food they are eating.
looks to me like a baby sea hare, it's so tiny
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.