Is this something that I should be concerned with?

[BradVerm]

Is this a bristle worm?

 

[Channas]

Yeah your common bristleworm, i personaly have nothing against them and see them as part of my cuc but many have had issues with them when they explode in numbers and they can bother corals. I have never had any issues with them though.

 

[saltwaterfish.com]

Yep 2nd vote for bristleworm, they are good detritivores for the tank. As long as there is not excess food around the population shouldn't get too big, just be careful sticking your bare hands in the sand or under rocks.