Okay, I 'm going to describe something I just found in my tank, and hopefully someone can tell me what it is.
It was crawling on a rock in a rather snail/slug like manner. The body is yellow, about one cm long, and about as big around as a pencil lead, and rather cylindrical (no bulge in the middle). At the front end, are two antennas that curl out and back in a U shape.
Along the body, are three (four?) rows of bright green spikes that stick up, about quarter to half a cm.
I have pulled it from the tank, into a holding dish, and will try to get a picture if it opens up again.