I just saw them this morning in a tank of curing Carribean Live Rock. They are small, flat, about 1/4-1/2", clearish white, with two "legs" . I see about a dozen of them.
I think--could be wrong--but I think those are tunicate worms. Harmless, scavengers. They build a tube, and those little legs are sticky, and used to pull food in.
They are smaller, 1/8" to 1/4", and in the center is a small hump that is more solidly colored. I figured they were harmless, but couldn't find an exact picture anywhere.