I believe there are literally hundreds of sub species that come under the term of Bristle Worm, so it's common to get some that look slit lay different. I have some that are a similar colour in my tank, but I also have some that are more pale in colour. Never been a problem for me.
Some people just don't like the way they look, etc, so if you have a search there are some guides on how to make traps for these guys if you want to get rid of them. Personally I think they add a little interest to the tank and help keep the detritus down.