What is this?

[Luckyyluciano]

Only comes out when the lights are off. Its about a inch long. Should I be concern?

 

[Jamo7]

Kinda looks like a tiny bristle worm. Hard to tell.

 

[Luckyyluciano]

Kinda hard to get a good picture

 

[James M]

bristle worm... good clean up crew

 

[OrionN]

It is indeed a bristle worm, great animal in a reef tank.