Man, I'm glad my flatworms arn't black/brown gross little bugs like that. Mine are tiny and white with a greenish tint. I havn't seen many of them in a few days, so maybe since I'm wet skimming they're falling back... who knows.
Good luck matt finding out what it is. I've got a ton of those silverbacked pods, and some zoos that came in on liverock have died, but Jessp's "pink death?" zoos are spreading like crazy, and I see new zoo polyps in my tank here and there, so who knows.