Here's a green worm I promised to post:
This was in the filter where I found the baby hermit crabs. It was barely a couple of mm. long, very green. It attracted my attention because it didn't move like most worms do, twisting and coiling and reversing direction every few seconds. This little guy went straight ahead, purposefully, to the edge of the drop of water, then along the side. Eventually, he began to push at the rim, distending it, stretching the drop out in the direction he wanted to go. I've seen flatworms do this, never other worms.
He had two distinct eyespots, and what looked like two smaller eyespots behind them. I think he has dark freckles along the sides, too.
All I can think of is a green ribbon worm, Emplectonema gracile, except that they grow to about 4 inches long; could this be a youngster? They have a lighter underside, which I couldn't see in this worm, as he was too small to be flipped over. They do have 10 to 20 eyespots on the head, fewer when they are tiny. (Description and photos, in Spanish.) They don't like the light; if this guy is E. gracile, I can see why he tried to push his way out of the drop of water under my lamp.
I put him in the tank; if he survives, I may see him again, grown up enough to identify properly.