Given that each tube seems to have two tentacles (called palps) sticking out, these are either Spionid or Phyllochaetopterus worms; based on the size of the palps, I'd guess Phyllochaetopterus, but they typically prefer to build their tubes down in the sand rather than the rock, so it's really 50-50 and they may be Spionids too (you'd probably need get a specimen out of its tube and under a microscope to tell for sure which it is).
Either way, generally speaking, both Spionids and Phyllochaetopterus worms (as long as they aren't popping up through/boring through your corals) are harmless/beneficial.
If they're reproducing quickly enough that you'd consider them pests, however, that's not good. These worms can be pretty difficult to eliminate when the need arises, as they're well-protected against predation and can regrow large chunks of their bodies (so you need to remove the whole worm, but they're quite difficult to remove). Currently, there are some forum members who are experimenting with various medications to try eliminating Coral-boring Spionids, and those experiments seem to have the only effective methods we've found so far for getting rid of them (though some species are vulnerable to less risky methods of removal such as coral dips).
Personally, I'd suggest trying to figure out what in your tank is allowing them to thrive and reproduce so quickly - such as if you have an excessive amount of tiny foods available/fed to the tank regularly. If you can find what's allowing them to thrive, you can see about changing it to control the population.
