You mentioned "Abalone" in your thread title, so I am assuming that in addition to looking like a slug, there is an abalone like shell.
Sounds like a
Stomatella sp. type snail, they look like they have teeney-tiny abalone shells.
They're awesome to have, they eat algae off your glass and stuff, and they are able reproduce in our tanks, which is cool.
Here is some additional reading, see if this is your mystery animal...
http://www.reefcorner.com/SpecimenSh...ella_varia.htm
Here's a picture from the
Reefs.org HitchHiker FAQ:
Does that look like your mystery hitchhiker?
I seem to remember reading somewhere once that supposedly they come in on rock from Indo...