What you are looking at are baby urchins, Tripneustes ventricosus, AKA the West Indian Sea Egg. While not typically seen in many reef aquariums (they are great reef cleaners btw), they are insanely popular under their alter ego Uni, the delicious orange/yellow sushi treat. Uni is also well known as an aphrodisiac, although your mileage might vary.
Master breeder and well known industry stalwart Martin Moe has been hard at work on rearing Diadema for years, and now it looks like he is having incredible success with the “Trips” as well, with over 300 juveniles in this batch. Martin is going to release a set of articles later this year documenting his progress. Stay tuned!