That was my impression, that they live on the chaeto, but eat detritus. That's why pod condos can be made of old sponges, piles or rock, or corrugated plastic. Like, you only need a couple to start the population, and it self regulates based on how much food there is. Pods come in on just about everything, frags, rocks, algae etc. The ultimate number you have is set by resources (how dirty your tank is) and predation (mandarins and wrasses seem especially effective at hunting them).