What the Heck is this?

  I have these little slug looking things running around my tank.  I don't know how many exactly, but I saw what I'm thinking was some reproductive activity yesterday and figure I better find out if these things are even something I should want in my tank.  As you can see by the pictures, there are different colored ones (they seem like they change color).  They also seem to have a sort of a oval shell on top of their backs.  I haven't seen them causing any problems, but I also don't want a million of them roaming around if they are a pest.  

  Thanks,

    Dan

Micah