If you see it again, I would try and remove it (at least to get a better pic/positive ID
). If it is a ribbon worm - the little I know about them indicates that they are not "reef friendly". AFAIK
, their diet consists mostly of other small invertebrates.
I'm not sure if you have, or have access to the book "The Modern Coral Reef Aquarium" - Vol. 3 - on pg. 91, there is a pic of a bright green ribbon worm (Parborlasia sp).
If you want to look up more about ribbon worms on the net, do a search for "Nemertea".
Also could be a Nematode ("round worm"):
More info. on these here: >>Nematodes - Ron Shimek<<