Slug, thing?

[Keen]

i remember seeing something on what you are talking about. i have 2 in my tank as well. they are algea eater, and are good for your tank. every once in a while you will see them spit out a milky white substance, which is their way of reproducing. in most developed tanks though, it will all get eaten by filter feeders and such, so reproduction probly wont happer. here is the site with all the info about them, with some pictures.
http://www.reefcorner.com/SpecimenSh...ella_varia.htm