Well that depends on alot of different things. If you can get any pictures for an ID that would be helpful, and about what size is it? For the most part people get hitchhikers that are >2" and freak about it killing everything in the tank, but a mantis that small can't really kill anything but the smaller members of your CUC. If you know where it lives (they have one den that they will live in, and close up at night), you can give it some krill or something, then it'll be even less likely to feel the need to hunt.