Having a lot of these in a tank typically means one of two things:
1. The tank is quite healthy.
or
2. The tank is being over-fed (usually not the case)
Over the years, I've found that most people experience a huge bloom in their popluation after they start feeding micro-foods such as phytoplankton. Also, this occurs if the tank has a low load of organisms that feed on them, and the tank is becoming stable. For example, a tank becoming healthy will bloom with all sorts of such critters in the absence of a population of reef-safe predatory crabs such as Mythrax, Arrows or others who like to munch on them. Same is true in absence of fish which prey on fanworms, etc. like angels and butterflys.
Assuming that your nitrates are where they should be, the presence of these likely means your tank is in good shape. A lot of these and high nitrates likely means weak skimming and an eco-system which is out of balance. FWIW.
Hope that helps.