I'm gonna vote for a sponge of some sort (assuming it's still there!)
If it were a baby urchin it would look just like a larger one, just TINY!
About 20 years ago I brought home a small coral from the LFS. I noticed what appeared to be a very small urchin on it - about BB sized. I've had it ever since. It is now nearly softball sized and happily comes out at night and scours my rocks of algae (both green and coraline). Its easy to follow it's path - there is a wide super clean section here or there. Probably the best freebie hitchhiker ever.