Harmless, reef safe filter feeders. If they start growing among your corals, they can irritate them and cause them to close up (like if they live between zoanthid polyps).
I got a ton in my tank and they do a great job of eating left overfood and poop. The only bad thing about the is they pile up the bigger pieces of sand on the top sandbeed as there bunker and I think it looks like crap.