its hard to tell from the picture, but now what you mention armored catepillar, i am going to say that it is a chiton of sorts. I have seen some get very large before, over three inches, as far as behavior goes, I really don't think that they are of any harm, but someone else might know for sure.
Now on the other hand, if it is fleshy and has no shell, then it may be a flatworm. In that case, get rid of it . . . not all flatworms are bad, but its hard to tell. Some can be a ridiculous nuisance. Trust me on that one . . .