Yeah, if your hitchhiker is one of those tiny white sea stars and then it very is likely Asterina. They are indeed excellent detritivores and have a negligible impact on the bioload in the tank unless their numbers get out of hand. There is quite a controversy in the reef community regarding whether or not the Asterina starfish are reef-safe animals are not. Some hobbyists feel they may prey on polyps and zoanthids, whereas Bob Fenner and many other reefers believe they only scavenge on dead or dying zoos and polyps, and are therefore beneficial inhabitants of the minireef. I’m inclined to believe that they are harmless and perform a beneficial service as detritivores.
I suspect the Asterina starfish in your nursery tank will be far more interested in the fecal pellets and droppings of your seahorse fry then in the little horselets themselves. I feel the little Asterinas only become problematic when their population explodes — they are very prolific and reproduce remarkably fast in the aquarium under favorable circumstances. So as long as your nursery tank doesn’t get overrun with them, I see no reason to remove the tiny starfish, Carrie. Just keep a close eye on him for any sign of predatory behavior in case the starfish is not actually an Asterina species but some other type of starfish.
In the meantime, you might also want to check out Bob Fenner’s FAQs on Asterina sea stars:
Click here: AsterinaFAQs
Best of luck rearing those babies, Carrie!