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Leslie Harries (comment to the last photo - user "Meerwasser") :"Your worms are amphinomidae. I have never received specimens of this type from aquarists for examination although it is the most common species found in reef tanks. They may be juveniles of Eurythoe complanata, another Eurythoe species, or belong to a different genus. They are not Hermodice or Pherecardia which are voracious predators on corals, crabs, worms, etc., as well as scavengers and which should be removed immediately. Eurythoe are primarily detritus feeders, herbivores, and scavengers. They will not eat healthy coral as far as I know but as scavengers they are attracted to diseased and rotting flesh so they will eat sick, wounded, or dying coral. As a result they are often blamed for coral mortality when they are only scavenging.
Much less is known about feeding in the other amphinomid genera. The small ones you have are primarily detritus & algae eaters based on what we know from reef tanks; I'm willing to bet that they are scavengers as well. As you've probably read, their numbers will go up if you overfeed a tank & decrease when the food supply goes down.
They should not hurt your corals. I say should not instead of won't because invertebrates don't read our reference books and don't know how they are supposed to act. There is always a possibility that if they are starving they will start eating things they don't normally eat . However, if they have been in your algae refuge for a while and your corals are healthy then they have proved themselves harmless under normal conditions."
Synoynms:
Amphinome jamaicensis Schmarda, 1861
Amphinome macrotricha Schmarda, 1861
Aphrodita complanata Pallas, 1766
Blenda armata Kinberg, 1867
Eurythoe albosetosa Kinberg, 1857
Eurythoe alcyonaria Gravier, 1902
Eurythoe assimilis McIntosh, 1925
Eurythoe capensis Kinberg, 1857
Eurythoe corallina Kinberg, 1857
Eurythoe ehlersi Kinberg, 1867
Eurythoe havaica Kinberg, 1867
Eurythoe indica Kinberg, 1867
Eurythoe kamehameha Kinberg, 1857
Eurythoe pacifica Kinberg, 1857
Eurythoe pacifica levukaensis McIntosh, 1885
Lycaretus neocephalicus Kinberg, 1867
Pleione alcyonea Savigny in Lamarck, 1818