Very interesting! I have no clue what you saw but I am going for optimism. Try squirting Cyclop-eze like you were target feeding coral in that spot tonight and see if it reappears. If you see it there again, try dangling a tiny piece of shrimp (on a thread or piece of dental floss maybe). Red and green are not normal colors for anything else I can think of. Was the green reflective/lumenescent? I have all manner of serpent stars and have never seen one red and green (green, yes and red yes but not both) and I have a couple of interesting LR night feeders that have arms that look like a baby serpent star but is likely some kind of worm (they never move and are mostly seen at night) but again, not in those colors so this is very exciting!
What color are the arms in the light?