Have you noticed any empty snail shells on the ground around the cave? My gut would tell me this guy isn’t safe for your inverts (or possibly coral), especially if it has the iridescent colors.Okay so im pretty sure i saw a white circle around its body near its head so i think it might be a
Eunice valens
which only grows to about 20cm
or i could be wrong cause i think its tentacles are dark/black colored but i gotta admit the colors this guy has which shines in all colors like a rainbow is nice too look at.