help identify please

[krrector]

I have noticed a few times these long worms in my tank that come out of the live rock. I am still trying to figure out what they are as I haven't found a good enough explanation that I am looking for. The one I am seeing right now doesn't come out all the way but I have seen at least an inch and a half of this thing. It appears to be black on top, white on the bottom. Appears to have like white stripes on it. At the head of it it seems like this thing comes out of it, would remind you of a feather duster coming in and out. I watched him for some time now and he takes this duster type thing and rubs it up and down the rock then pulls it back in. Weird thing let me tell you. Anyone know what it is?

 

[mlm]

I don't know the proper name of it but I do have a few of what you descride in my tank. They seem to only come out at night eat dig in the sand for detritus.

 

[krrector]

I need to know if they are good or bad for a tank. Because now that I know where he is, might be the opportunity to get it out before he goes somewhere else. Thanks for all replies

 

[mlm]

I would sat that anything that eats detritus is a good thing.