Help to identify -- what the heck is this slug looking thing?

[akjayintx]

I have two of these that must have come with the LR....One is dark, the other is light. They don't seem to be causing any harm but they have grown very fast from teeny little things to about one inch to an inch and a half in length... They aren't very attractive but they seem to eat a lot of detrius and algae. I just want to make sure they won't harm anything else I put in the tank in the future... I haven't quite mastered this photographing the aquarium thing so sorry its kinda blurry and bright.