Harmless scavengers. Most "bristleworms" we find in our tanks are harmless. You only need to watch out for the polychaete Hermodice carunculata which is rare to find. Those ones pictured are good detritus eaters.
20 Gallon SPS reef
29 Gallon FW "fat catfish" tank
20 Gallon Long Dart Frogs "Zig" and "Zag"