Most bristleworms (which is what you have) are pretty benign and aren't a bad addition to your cleaning crew since they're scavengers/detritus eaters. Often when people see them feeding on a dead fish or invert they assume the worms killed them, when most likely it was already dead.
You don't, however, want them overrunning your tank, so you can prevent that by not overfeeding or overstocking. You may also want to remove the visible ones.
Even so, there are some species that may harm your inhabitants, So it would be helpful if you could get a good picture for ID.
Regarding the possible shrimp, can't really answer any questions about those without a positive ID, so try to get pics or at least a really good description.