There are many many different species of what hobbyists refer to as "bristleworms". 99.9999999% of them are harmless, not only harmless, but beneficial detrivores.
Some get bigger than others the typical range is from tiny up to 4" or thereabouts. As for whether or not your fish will eat them - usually not, but it depends on the type of fish. Some wrasses and some pseudochromis are known for preying on them.