Hitchhiking slugs?! A suprise!

I woke this AM, and looked in the tank as I have since I began cycling(6wks ago)). To my suprise, there was life in the tank(other than what i've added) all over my rocks. . . .slugs.. that's what they seem to be. i must have counted at least 15 w/o seeing behind the rockwork. They are all no bigger than about a 1/4'' long, and 3/16" wide... most are black w/ 1 white dot on their tops.....the others are nearly the same color as the fiji rock(greenish/whitish, and almost translucent). they all can move pretty well, too!
So, can anyone tell me exactly what species they are, and if there is anything that I need to do to satisfy them? They must be nocturnal, cause I can't find them now... I wonder why I am only now seeing them. . .

 

pics will help

 

I'll get the pics if I catch them out again.